Bruce
01-22-2004, 07:31 AM
I have some editing to do to allow for this to include the MAF codes, but for the 749 ecm, and as a quick reference it'll due for a while.
BTW, when you have more then one code, you start diagnosing the problem with the highest number first.
CODE 12
NO DISTRIBUTOR REFERENCE PULSE
KEY is on, but engine is not running.
CODE 14
COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT
(HIGH TEMPEATURE INDICATED)
CIRCUIT DESCRIPTION:
The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less and the voltage
drops. At normal engine operating temperature, the voltage will measure
about 1.5 to 2.0 volts at the ECM terminal "GE 16."
Coolant temperature is one of the inputs used to control:
Fuel Delivery
Torque Converter Clutch (TCC)
Idle Air Control (IAC)
Coolant Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks to see if code was set as result of hard failure or intermittent
condition. Code 14 will set if:
Engine has been running for more than 10 seconds.
Signal voltage indicates a coolant temperature above 135'C (275'F) for 3
seconds.
2. This test simulates conditions for a Code 15. If the ECM recognizes the
open circuit.
CODE 15
COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT
(LOW TEMPEATURE INDICATED)
CIRCUIT DESCRIPTION:
The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on CKT 410 to the sensor When the engine is cold, the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less and the voltage
drops. At normal engine operating temperature, the voltage will measure
about 1.5 volts to 2.0 volts at the ECM terminal "GE 16."
Coolant temperature is one of the inputs used to control:
Fuel Delivery
Torque Converter Clutch (TCC)
Idle Air Control (IAC)
Coolant Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks to see if code was set as result of hard failure or intermittent
condition. Code 15 will set if:
Engine has been running for more than 50 seconds.
Signal voltage indicates a coolant temperature below -30'C (-22'F).
2. This test simulates conditions for a Code 14. If the ECM recognizes the
grounded circuit (low voltage) and displays a high temperature, the ECM and
wiring are OK.
3. This test will determine if there is a wiring problem or a faulty ECM. If
CKT 452 is open, there may also be a Code 21 stored.
DIAGNOSTIC AIDS:
A "Scan" tool reads engine temperature in degrees centigrade. After the
engine is started, the temperature should rise steadily to about 90'C
(194'F), then stabilize when the thermostat opens. If the engine has been
allowed to cool to an ambient temperature (overnight), coolant and MAT
temperature may be checked with a "Scan" tool and should read close to each other. When a Code 15 is set, the ECM will turn "ON" the engine cooling fan.
A Code 15 will result if CKTs 410 or 452 are open. If Code 15 is
intermittent, refer to Intermittents page.
CODE 21
THROTTLE POSITION SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE HIGH)
CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes
relative to the throttle valve. Signal voltage will vary from less than 1.0
volt at idle to about 4.6 volts at wide open throttle (WOT).
The TPS signal is one of the most important inputs used by the ECM for fuel
control and for many of the ECM controlled outputs.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step checks to see if Code 21 is the result of a hard failure or an
intermittent condition. A Code 21 will set if:
TPS reading above 2.5 volts
MAP reading below 70 kPa (M/T) or 81 kPa (A/T)
Engine speed less than 1300 rpm
All of the above conditions are present for 10 seconds
2. This step simulates conditions for a Code 22. If the ECM recognizes the
change of state, the ECM and CKTs 416 and 417 are OK.
3. This step isolates a faulty sensor, ECM, or an open CKT 155.
DIAGNOSTIC AIDS:
A "Scan" tool displays throttle position in volts. Closed throttle voltage
should be less than 1.0 volt. TPS voltage should increase at a steady rate
as throttle is moved to WOT.
A Code 21 will result if CKT 155 is open or CKT 417 is shorted to voltage.
If Code 21 is intermittent, refer to Intermittents page.
CODE 22
THROTTLE POSITION SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE LOW)
CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes
rclative to the throttle valve. Signal voltage will vary from less than 1.0
volt at idle to about 4.5 volts at wide open throttle (WOT).
The TPS signal is one of the most important inputs used by the ECM for fuel
control and for many of the ECM controlled outputs.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 22 will set if:
Engine is running
TPS signal voltage is less than .2 volt for 4 seconds
2. Simulates Code 21: (high voltage). If ECM recognizes the high signal
voltage the ECM and wiring are OK.
3. With closed throttle, ignition "ON" or at idle, voltage at "GF13" should
be .36-.44 volt. If not, replace the TPS.
4. Simulates a high signal voltage. Checks CKT 417 for an open.
DIAGNOSTIC AIDS:
A "Scan" tool reads throttle position in volts. Voltage should increase at a
steady rate as throttle is moved toward WOT. Also some "Scan" tools will
read throttle angle 0%=closed throttle, l00% = WOT. An open or short to
ground in CKTs 416 or 417 will result in a Code 22.
Poor Connection or Damaged Harness. Inspect ECM harness connectors for
backed out terminal "GF13," improper mating, broken locks, improperly formed
or damaged terminals, poor terminal to wire connection, and damaged harness.
Intermittent Test. If connections and harness check OK, monitor TPS voltage display while moving related connectors and wiring harness. If the failure is induced, the display will change. This may help to isolate the location of the malfunction.
TPS Scaling. Observe TPS voltage display while depressing accelerator pedal
with engine stopped and ignition "ON." Display should vary from closed
throttle TPS voltage when throttle was closed, to over 4.5 volts (4500 mV)
when throttle is held at wide open throttle position.
CODE 23
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
CIRCUIT DESCRIPTION:
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the
signal voltage to the ECM The ECM applies a voltage (about 5 volts) on CKT
472 to the sensor. When the air is cold the sensor (thermistor) resistance
is high, therefore the ECM will see a high signal voltage. If the air is
warm, the sensor resistance is l( therefore, the ECM will detect a low
voltage.
MAT Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. Code 23 will
set if:
A signal voltage indicates a manifold air temperature below - 30'C (-22'F).
Boost conditions have been present for longer than 10 seconds.
Due to the conditions necessary to set a Code 23, the "Check Engine" light
will only stay "ON" while the fault is present.
1. A "Scan" tool may not be used to diagnose this fault, due to the ECM
transmitting "default" (substitute) values while the fault is present. A
Code 23 will set due to an open sensor, wire, or connection. This test will
determine if the wiring and ECM are OK.
2. If the resistance is greater than 25,000 ohms, replace the sensor.
CODE 24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
CIRCUIT DESCRIPTION:
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the
DRAC, which alternately grounds CKT 437, when receiving voltage pulses from Vehicle Speed Sensor (VSS) when drive wheels are turning. This pulsing
action takes place about 2000 times per mile and the ECM will calculate
vehicle speed based on the time between "pulses."
A "Scan" tool reading should closely match the speedometer reading with
drive wheels turning.
TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Code 24 will set if:
CKT 437 voltage is constant.
Engine speed is more than 1200 rpm.
Vehicle speed signal indicates less than 2 mph (3 km/h) on Tech 1.
Not in park or neutral.
All conditions must be met for 5 seconds.
** These conditions are met during a road load deceleration.
2. This test determines if the DRAC is receiving the A/C signal from the
VSS.
3. This test monitors the DRAC voltage on CKT 437. With the wheels turning,
the pulsing action will result in a varying voltage. The variation will be
greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32
km/h).
DIAGNOSTIC AIDS:
1. "Scan" reading should closely match speedometer reading, with drive
wheels turning.
2. Check Park/Neutral (P/N) switch diagnosis chart.
3. If Park/Neutral (P/N) switch is OK, refer to Intermittents page.
CODE 25
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(HIGH TEMPERATURE INDICATED)
TEST DESCRIPTION:
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the
signal voltage to the ECM The ECM applies a voltage (4-6 volts) on CKT 472
to the sensor. When manifold air is cold, the sensor (thermistor) resistance
is high, therefore the ECM will detect a high signal voltage. If the air
warms, the sensor resistance becomes less, and the voltage drops.
NOTE: A "Scan" tool may not be used to diagnose this fault, due to the ECM
transmitting "Default" (Substitute) values when the fault is present.
MAT Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. Code 25 will
set if the engine is not experiencing turbocharger boost and the following
conditions are met:
Signal voltage indicates a manifold air temperature greater than 135'C
(275'F).
The above requirement is met for at least 30 seconds.
Due to the conditions necessary to set a Code 25, the "Check Engine" light
will only stay "ON" while the fault is present.
1. A "Scan" tool may not be used to diagnose this fault due to the ECM
transmitting "default" (substitute) values while the fault is present. If
voltage is above 4 volts, the ECM and wiring are OK.
2. If the resistance is less than 100 ohms, replace the sensor.
CODE 31
TURBO WASTEGATE OVERBOOST
CIRCUIT DESCRIPTION:
On turbo charged engines, the exhaust gases pass from the exhaust manifold
through the turbocharger, turning the turbine blades. The compressor side of
the turbocharger also turns, pulling air through the air filter and pushing
the air into the intake manifold, pressurizing the intake manifold.
The wastegate is normally closed, but opens to bypass exhaust gas to prevent an overboost condition. The wastegate will open when p ressure is a pp lied to the actuator, and is controlled by a wastegate control solenoid valve pulsed "ON" and" OFF" by the E CM. Under normal driving conditions, the control solenoid is energized all the time which closes "OFF" the manifold
pressure to the wastegate actuator. This allows for a rapid increase in
boost pressure. A boost increase will be detected by the MAP sensor, and the ECM will pulse the wastegate control valve. Manifold pressure will then be
allowed to pass to the wastegate actuator, and the actuator will open the
wastegate This will prevent an overboost condition on heavy acceleration. As
boost pressure decreases, the ECM closes the control valve and the wastegate actuator pressure bleeds "OFF" through the vent in the control valve. If an overboost does exist as indicated by the MAP sensor, the ECM will reduce fuel delivery to prevent damage to the engine.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. A Code 31 will set when the manifold absolute pressure exceeds 205 kPa of boost for two seconds, and a Code 33 has not previously been set. Code 31 will set, but the "Cheek Engine" light will stay "ON" only while the
overboost exists. The light will stay "ON" for 10 seconds after the
condition exists and then go "OUT."
An overboost condition could be caused by:
CKT 471 shorted to ground
A sticking wastegate actuator or wastegate
A control valve stuck in the closed position
A cut or pinched hose
A faulty ECM
An extremely dirty air filter
** With ignition shut "OFF," the control valve solenoid is open.
2. After the 103 kPa (15 psi) is applied to valve and then the pressure
source is removed, the actuator should slowly move back and close the
wastegate. If the pressure does not bleed "OFF," the vent in the control
valve solenoid could be plugged.
3. With the ignition "ON" and the diagnostic terminal grounded, the' control
valve solenoid should be energized. This closes "OFF" the manifold to the
wastegate actuator.
4. Check the electrical control portion of the system. With key "ON" and
engine not running, the solenoid should not be energized.
CODE 32
EXHAUST GAS RECERCULATION (EGR) CIRCUIT
CIRCUIT DESCRIPTION:
The ECM operates a vacuum solenoid to control the Exhaust Gas Recirculation
(EGR) valve. This solenoid is normally closed. By providing a ground path,
the ECM energizes the solenoid, allowing vacuum to reach the EGR valve.
Under certain conditions, when the EGR valve is normally open, the ECM tests
the EGR function by de-energizing the EGR control solenoid, blocking vacuum
to the EGR valve diaphragm. Without EGR, the system will sense a lean
condition and will increase the fuel integrator rate in response. The ECM
monitors the amount of fuel delivery increase. If the increase is below a
specified value, the ECM will interpret that the test was failed. The
failure indicates that closing the EGR valve when it would normally be open
does not make a significant change, indicating a problem in the EGR system.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. The
diagnostic chart covers checks for the entire EGR system. If no trouble is
found but Code 32 was set, an intermittent electrical condition or a sticky
EGR valve is at fault.
DIAGNOSTIC AIDS:
The vacuum switch in the EGR solenoid assembly is not used.
An EGR valve stuck open will cause a rough idle.
A plugged EGR solenoid vent filter could cause the EGR valve to remain open
or to close slowly.
An inoperative check valve in the ported vacuum line will result in faulty
EGR system operation.
CODE 33
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT
(SIGNAL VOLTAGE HIGH - LOW VACUUM)
CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold
pressure (vacuum). The ECM receives this information as a signal voltage
that will vary from about l-l.5 volts at closed throttle idle, to 4-4.5
volts at wide open throttle (low vacuum or boost).
If the MAP sensor fails, the ECM will substitute a fixed MAP value and use
the Throttle Position Sensor (TPS) to control fuel delivery.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step will determine if Code 33 is the result of a hard failure or an
intermittent condition.
A Code 33 will set if:
MAP signal voltage is too high (low vacuum)
TPS less than 2%
These conditions for a time longer than 5 seconds
2. This step simulates conditions for a Code 34. If the ECM recognizes the
change, the ECM and CKTs 474 and 432 are OK. If CKT 452 is open, there may
also be a stored Code 23.
DIAGNOSTIC AIDS:
With the ignition "ON" and the engine stopped, the manifold pressure is
equal to atmospheric pressure and the signal voltage will be high. This
information is used by the ECM as an indication of vehicle altitude and is
referred to as BARO. Comparison of this BARO reading with a known good
vehicle with the same sensor is a good way to check accuracy of a "suspect"
sensor. Reading should be the same +-.4 volt.
A Code 33 will result if CKT 452 is open, or if CKT 432 is shorted to
voltage or to CKT 474. If Code 33 is intermittent, refer to Intermittents
page.
CODE 34
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT
(SIGNAL VOLTAGE LOW - HIGH VACUUM)
CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold
pressure (vacuum). The ECM receives this information as a signal voltage
that will vary from less than 1.0 volt at closed throttle idle, to 4-4.5
volts at wide open throttle.
If the MAP sensor fails, the ECM will substitute a fixed MAP value and use
the Throttle Position Sensor (TPS) to control fuel delivery.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step determines if Code 34 is the result of a hard failure or an
intermittent condition.
A Code 34 will set when:
MAP signal voltage is too low
Engine speed below 1200 rpm and/or TPS greater than 20%
2. Jumpering harness terminals "B" to "C", 5 volts to signal, will determine
if the sensor is at fault or if there is a problem with the ECM or wiring.
3. The "Scan" tool may not display battery voltage. The important thing is
that the ECM recognizes the voltage as more than 4 volts, indicating that
the ECM and CKT 432 are OK.
DIAGNOSTIC AIDS:
With the ignition "ON" and the engine stopped, the manifold pressure is
equal to atmospheric pressure and the signal voltage will be high. This
information is used by the ECM as an indication of vehicle altitude and is
referred to as BARO. Comparison of this BARO reading with a known good
vehicle with the same sensor is a good way to check accuracy of a "suspect"
sensor. Reading should be the same +-0.4 Volt.
A Code 34 will result if CKTs 474 or 432 are open or shorted to ground.
If CKT 416 is shorted to ground, there may also be a stored Code 22.
If Code 34 is intermittent, refer to Intermittents page.
CODE 35
IDLE SPEED ERROR
CIRCUIT DESCRIPTION:
The ECM controls engine idle speed with the IAC valve. To increase idle
speed, the ECM retracts the IAC valve pintle away from its seat, allowing
more air to bypass the throttle bore. To decrease idle speed, it extends the
IAC valve pintle towards its seat, reducing bypass air flow. A "Scan" tool
will read the ECM commands to the IAC valve in counts. Higher the counts
indicate more air bypass (higher idle). The lower the counts indicate less
air is allowed to bypass (lower idle). Code 35 will set when the closed
throttle engine speed is 225 rpm above or below the desired (commanded) idle
speed for 20 seconds.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. The Tech 1 rpm control mode is used to extend and retract the IAC valve.
The valve should move smoothly within the specified range. If the idle speed
is commanded (IAC extended) too low (below 700 rpm), the engine may stall.
This may be normal and would not indicate a problem. Retracting the IAC
beyond its controlled range (above 1500 rpm) will cause a delay before the
rpm's start dropping. This too is normal.
2. This test uses the Tech 1 to command the IAC controlled idle speed. The
ECM issues commands to obtain commended idle speed. The node lights each
should flash red and green to indicate a good circuit as the ECM issues
commands. While the sequence of color is not important if either light is
"OFF" or does not flash red and green, check the circuits for faults,
beginning with poor terminal contacts.
DIAGNOSTIC AIDS:
A slow, unstable, or fast idle may be caused by a non--IAC system problem
that cannot be overcome by the IAC valve. Out of control range IAC "Scan"
tool counts will be above 60 if idle is too low, and zero counts if idle is
too high. The following checks should be made to repair a non-IAC system
problem:
Vacuum Leak (High Idle)
If idle is too high, stop the engine. Fully extend (low) IAC with tester.
Start engine. If idle speed is above 800 rpm, locate and correct vacuum leak
including PCV system. Also check for binding of throttle blade or linkage.
System too lean (High Air/Fuel Ratio)
The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. Code 44 may be set. "Scan 02 voltage will be less than 300 mV (.3 volts). Check for low regulated fuel pressure, water in the fuel or a restricted injector.
System too rich (Low Air/Fuel Ratio)
The idle speed will be too low. "Scan" tool IAC Counts will usually be above
80. System is obviously rich and may exhibit black smoke in exhaust.
"Scan" tool O2 voltage will be fixed above 800 mV (.8 volt). Check for high
fuel pressure, leaking or sticking injector. Silicone contaminated O2 "Scan"
voltage will be slow to respond.
Throttle Body
Remove IAC valve and inspect bore for foreign material.
IAC Valve Electrical Connections
IAC valve connections should be carefully checked for proper contact.
PCV Valve
An incorrect or faulty PCV valve may result in an incorrect idle speed.
Refer to "Rough, Unstable, Incorrect Idle or Stalling" in "Symptoms," in
Section "B."
If intermittent poor driveability or idle symptoms are resolved by
disconnecting the IAC, carefully recheck connections, valve terminal
resistance, or replace IAC
CODE 42
ELECTRONIC SPARK TIMING (EST) CIRCUIT
CIRCUIT DESCRIPTION:
The ignition module sends a reference signal (CKT 430) to the ECM when the
engine is cranking. While the engine speed is under 400 rpm, the ignition
module will control ignition timing. When the engine speed exceeds 400 rpm,
the ECM applies 5 volts to the bypass line (CKT 424) to switch the timing to
ECM control (EST CKT 423).
When the system is running "ON" the ignition module, that is, no voltage on
the bypass line, the ignition module grounds the EST signal. The ECM expects
to see no voltage on the EST line during this condition. If it sees a
voltage, it sets Code 42 and will not go into the EST mode.
When the rpm for EST is reached (about 400 rpm), voltage will be applied to
the bypass line, the EST should no longer be grounded in the ignition
module, so the EST voltage should be varying.
If the bypass line is open or grounded, the ignition module will not switch
to EST mode, so the EST voltage will be low and Code 42 will be set.
If the EST line is grounded, the ignition module will switch to EST but,
because the line is grounded, there will be no EST signal. A Code 42 will be
set.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Confirms Code 42 and that the fault ca using the code is present.
2. Checks for a normal EST ground path through the ignition module. An EST
CKT 423 shorted to ground will also read less than 500 ohms, however, this
will be checked later.
3. As the test light voltage touches terminal "C7," the module should
switch, causing the ohmmeter to "overrange" if the meter is in the 1000~2000 ohms position. Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
4. The module did not switch and this step checks for:
EST CKT 423 shorted to ground
Bypass CKT 424 open
Faulty ignition module connection or module
5. Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423
or 424.
CODE 43
ELECETRONIC SPARK CONTROL (ESC) CIRCUIT
CIRCUIT DESCRIPTION:
The knock sensor is used to detect engine detonation and the ECM will retard
the electronic spark timing based on the signal being received. The
circuitry within the knock sensor causes the ECM 5 volts to be pulled down
so that under a no knock condition, CKT 496 would measure about 2.5 volts.
The knock sensor produces an A/C signal which rides on the 2.5 volts, DC
voltage. The amplitude and signal frequency is dependent upon the knock
level.
If CKT 496 becomes open or shorted to ground the voltage will either go
above 3.5 volts or below 1.5 volts. If either of these conditions are met
for about 5 seconds, a Code 43 will be stored.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 43 will set when:
Coolant temperature is over 90'C
MAT temperature is over O'C
High engine load based on MAP and rpm
Voltage on CKT 496 goes above 3.5 volts or below 1.5 volts
All conditions present for 5 seconds
If an audible knock is heard from the engine, repair the internal engine
problem, as normally no knock should be detected at idle.
2. If tapping on the engine lift hook does not produce a knock signal, try
tapping engine closer to sensor before proceeding.
3. The ECM has a 5 volts pull-up resistor, which should be present at the
knock sensor terminal.
4. This test determines if the knock sensor is faulty or if the ESC portion
of the Mem-Cal is faulty.
DIAGNOSTIC AIDS:
Check CKT 496 for a potential open or short to ground. Also check for proper
installation of Mem-Cal
Refer to the Intermittents page.
CODE 44
OXYGEN (O2) SENSOR CIRCUIT
(LEAN EXHAUST INDICATED)
CIRCUIT DESCRIPTION:
The ECM applies a bias voltage of approximately 450 millivolts (350-550 mV
is normal bias voltage) between terminals "GEI4" and "GElS." The oxygen (02)
sensor varies the voltage within a range of about 1 volt, if the exhaust is
rich, down through about .10 volt, if exhaust is lean. As each pair of
exhaust valves open, a different exhaust oxygen content passes by the oxygen sensor, which causes the oxygen sensor voltage output to vary. A lean exhaust condition will cause the oxygen sensor to output a low voltage,
which will pull the bias voltage from CKT412 low. The ECM is programmed to
interpret any voltage less than 500 mV as a "lean exhaust condition."
The sensor is like an open circuit and produces no voltage when it is below
about 360'C (680'F). An open sensor circuit causes "Open Loop" operation.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This test checks the oxygen sensor's heating element. The heating element
resistance should be 3.5 ohms at 20'C (68'F) or 14 ohms at 350'C (562'F).
2. Code 44 is set when the oxygen (02) sensor signal voltage on CKT 412;
Remains below .2 volt for 2 minutes and the fuel system is operating in
"Closed Loop."
DIAGNOSTIC AIDS:
Using the Tech I diagnostic computer "Scan" tool, observe the block learn
values at different rpm and air flow conditions. The "Scan" tool also
displays the block cells, so the block learn values can be checked in each
of the cells to determine when the Code 44 may have been set. If the
conditions for Code 44 or Code 64 exist, the block learn values will be
around 150.
O2 Sensor Wire. Sensor pigtail may be mispositioned and contacting the
exhaust manifold.
Check for intermittent ground in wire between connector and sensor.
Poor connection at oxygen (02) sensor ground wire.
Lean Injector(s).
Fuel Contamination. Water, even in small amounts, near the in-tank fuel
pump inlet can be delivered to the injectors. The water causes a lean
exhaust and can set a Code 44 or Code 64.
Fuel Pressure. System will be lean if pressure is too low. It may be
necessary to monitor fuel pressure while driving the car at various road
speeds and/or loads to confirm.
Exhaust Leaks. If there is an exhaust leak, the engine can cause outside
air to be pulled into the exhaust and past the sensor. Vacuum or crankcase
leaks can cause a lean condition.
If the above are OK, it is a faulty oxygen sensor.
CODE 45
OXYGEN (O2) SENSOR CIRCUIT
(RICH EXHAUST INDICATED)
CIRCUIT DESCRIPTION:
The ECM supplies a voltage of about .45 volt between terminals "GE 14" and
"GE 15". (If measured with a 10 megohm digital voltmeter, this may read as
low as .32 volt.) The O2 sensor varies the voltage within a range of about 1
volt if the exhaust is rich, down through about .10 volt if exhaust is lean.
The sensor is like an open circuit and produces no voltage when it is below
about 316'C (600'F). An open sensor circuit causes "Open Loop" operation.
The heating element in the O2 sensor causes the sensor to heat up quickly,
allowing for quicker closed-loop operation.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 45 is set when the O2 sensor voltage:
Remains above .75 volt for 50 seconds; and the system is in "Closed Loop."
DIAGNOSTIC AIDS:
Using the "Scan," observe the block learn values at different rpm and air
flow conditions. The "Scan" also displays the block cells, so the block
learn values can be checked in each of the ceHs to determine when the Code
45 may have been set. If the conditions for Code 45 exists, the block learn
values witor)l be around 115.
Fuel Pressure. System will go rich if pressure is too high. The ECM can
compensate for some increase. However, if it gets too high, a Code 45 may be set.
Rich Injector. Perform injector balance test.
Leaking Injector.
Check for fuel contaminated oil.
HEI Shielding. An open ignition ground CKT 453 may result in EMI, or
induced electrical "noise." The ECM looks at this "noise" as reference
pulses. The additional pulses result in a higher than actual engine speed
signal. The ECM then delivers too much fuel, causing system to go rich.
Engine tachometer will also show higher than actual engine speed, which can
help in diagnosing this problem.
Canister purge. Check canister for fuel saturation. If full of fuel, check
canister control and hoses. See "Purge Valve Operation" Section "C3."
MAP Sensor. An output that causes the ECM to sense a higher than normal
manifold pressure can cause the system to go rich. Disconnecting the MAP
sensor will allow the ECM to set a fixed value for the sensor. Substitute a
different MAP sensor if the rich condition is gone while the sensor is
disconnected.
Check for leaking fuel pressure regulator diaphragm by checking vacuum line
to regulator for fuel.
TPS. An intermittent TPS output will cause the system to go rich, due to a
false indication of the engine accelerating.
EGR. An EGR staying open (especially at idle) will cause the O2 sensor to
indicate a rich exhaust, and this could result in a Code 45
CODE 51
PROM ERROR
(FAULTY OR INCORRECT PROM)
TEST DESCRIPTION:
Check that all pins are fully inserted in the socket and that PROM is
properly seated. If OK, replace PROM, clear memory, and recheck.
If Code 51 reappears, replace ECM.
BTW, when you have more then one code, you start diagnosing the problem with the highest number first.
CODE 12
NO DISTRIBUTOR REFERENCE PULSE
KEY is on, but engine is not running.
CODE 14
COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT
(HIGH TEMPEATURE INDICATED)
CIRCUIT DESCRIPTION:
The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM The ECM applies a voltage on CKT 410 to the sensor. When the engine is cold, the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less and the voltage
drops. At normal engine operating temperature, the voltage will measure
about 1.5 to 2.0 volts at the ECM terminal "GE 16."
Coolant temperature is one of the inputs used to control:
Fuel Delivery
Torque Converter Clutch (TCC)
Idle Air Control (IAC)
Coolant Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks to see if code was set as result of hard failure or intermittent
condition. Code 14 will set if:
Engine has been running for more than 10 seconds.
Signal voltage indicates a coolant temperature above 135'C (275'F) for 3
seconds.
2. This test simulates conditions for a Code 15. If the ECM recognizes the
open circuit.
CODE 15
COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT
(LOW TEMPEATURE INDICATED)
CIRCUIT DESCRIPTION:
The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on CKT 410 to the sensor When the engine is cold, the sensor (thermistor) resistance is high, therefore, the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less and the voltage
drops. At normal engine operating temperature, the voltage will measure
about 1.5 volts to 2.0 volts at the ECM terminal "GE 16."
Coolant temperature is one of the inputs used to control:
Fuel Delivery
Torque Converter Clutch (TCC)
Idle Air Control (IAC)
Coolant Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Checks to see if code was set as result of hard failure or intermittent
condition. Code 15 will set if:
Engine has been running for more than 50 seconds.
Signal voltage indicates a coolant temperature below -30'C (-22'F).
2. This test simulates conditions for a Code 14. If the ECM recognizes the
grounded circuit (low voltage) and displays a high temperature, the ECM and
wiring are OK.
3. This test will determine if there is a wiring problem or a faulty ECM. If
CKT 452 is open, there may also be a Code 21 stored.
DIAGNOSTIC AIDS:
A "Scan" tool reads engine temperature in degrees centigrade. After the
engine is started, the temperature should rise steadily to about 90'C
(194'F), then stabilize when the thermostat opens. If the engine has been
allowed to cool to an ambient temperature (overnight), coolant and MAT
temperature may be checked with a "Scan" tool and should read close to each other. When a Code 15 is set, the ECM will turn "ON" the engine cooling fan.
A Code 15 will result if CKTs 410 or 452 are open. If Code 15 is
intermittent, refer to Intermittents page.
CODE 21
THROTTLE POSITION SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE HIGH)
CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes
relative to the throttle valve. Signal voltage will vary from less than 1.0
volt at idle to about 4.6 volts at wide open throttle (WOT).
The TPS signal is one of the most important inputs used by the ECM for fuel
control and for many of the ECM controlled outputs.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step checks to see if Code 21 is the result of a hard failure or an
intermittent condition. A Code 21 will set if:
TPS reading above 2.5 volts
MAP reading below 70 kPa (M/T) or 81 kPa (A/T)
Engine speed less than 1300 rpm
All of the above conditions are present for 10 seconds
2. This step simulates conditions for a Code 22. If the ECM recognizes the
change of state, the ECM and CKTs 416 and 417 are OK.
3. This step isolates a faulty sensor, ECM, or an open CKT 155.
DIAGNOSTIC AIDS:
A "Scan" tool displays throttle position in volts. Closed throttle voltage
should be less than 1.0 volt. TPS voltage should increase at a steady rate
as throttle is moved to WOT.
A Code 21 will result if CKT 155 is open or CKT 417 is shorted to voltage.
If Code 21 is intermittent, refer to Intermittents page.
CODE 22
THROTTLE POSITION SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE LOW)
CIRCUIT DESCRIPTION:
The Throttle Position Sensor (TPS) provides a voltage signal that changes
rclative to the throttle valve. Signal voltage will vary from less than 1.0
volt at idle to about 4.5 volts at wide open throttle (WOT).
The TPS signal is one of the most important inputs used by the ECM for fuel
control and for many of the ECM controlled outputs.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 22 will set if:
Engine is running
TPS signal voltage is less than .2 volt for 4 seconds
2. Simulates Code 21: (high voltage). If ECM recognizes the high signal
voltage the ECM and wiring are OK.
3. With closed throttle, ignition "ON" or at idle, voltage at "GF13" should
be .36-.44 volt. If not, replace the TPS.
4. Simulates a high signal voltage. Checks CKT 417 for an open.
DIAGNOSTIC AIDS:
A "Scan" tool reads throttle position in volts. Voltage should increase at a
steady rate as throttle is moved toward WOT. Also some "Scan" tools will
read throttle angle 0%=closed throttle, l00% = WOT. An open or short to
ground in CKTs 416 or 417 will result in a Code 22.
Poor Connection or Damaged Harness. Inspect ECM harness connectors for
backed out terminal "GF13," improper mating, broken locks, improperly formed
or damaged terminals, poor terminal to wire connection, and damaged harness.
Intermittent Test. If connections and harness check OK, monitor TPS voltage display while moving related connectors and wiring harness. If the failure is induced, the display will change. This may help to isolate the location of the malfunction.
TPS Scaling. Observe TPS voltage display while depressing accelerator pedal
with engine stopped and ignition "ON." Display should vary from closed
throttle TPS voltage when throttle was closed, to over 4.5 volts (4500 mV)
when throttle is held at wide open throttle position.
CODE 23
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
CIRCUIT DESCRIPTION:
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the
signal voltage to the ECM The ECM applies a voltage (about 5 volts) on CKT
472 to the sensor. When the air is cold the sensor (thermistor) resistance
is high, therefore the ECM will see a high signal voltage. If the air is
warm, the sensor resistance is l( therefore, the ECM will detect a low
voltage.
MAT Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. Code 23 will
set if:
A signal voltage indicates a manifold air temperature below - 30'C (-22'F).
Boost conditions have been present for longer than 10 seconds.
Due to the conditions necessary to set a Code 23, the "Check Engine" light
will only stay "ON" while the fault is present.
1. A "Scan" tool may not be used to diagnose this fault, due to the ECM
transmitting "default" (substitute) values while the fault is present. A
Code 23 will set due to an open sensor, wire, or connection. This test will
determine if the wiring and ECM are OK.
2. If the resistance is greater than 25,000 ohms, replace the sensor.
CODE 24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
CIRCUIT DESCRIPTION:
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the
DRAC, which alternately grounds CKT 437, when receiving voltage pulses from Vehicle Speed Sensor (VSS) when drive wheels are turning. This pulsing
action takes place about 2000 times per mile and the ECM will calculate
vehicle speed based on the time between "pulses."
A "Scan" tool reading should closely match the speedometer reading with
drive wheels turning.
TEST DESCRIPTION:
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Code 24 will set if:
CKT 437 voltage is constant.
Engine speed is more than 1200 rpm.
Vehicle speed signal indicates less than 2 mph (3 km/h) on Tech 1.
Not in park or neutral.
All conditions must be met for 5 seconds.
** These conditions are met during a road load deceleration.
2. This test determines if the DRAC is receiving the A/C signal from the
VSS.
3. This test monitors the DRAC voltage on CKT 437. With the wheels turning,
the pulsing action will result in a varying voltage. The variation will be
greater at low wheel speeds to an average of 4-6 volts at about 20 mph (32
km/h).
DIAGNOSTIC AIDS:
1. "Scan" reading should closely match speedometer reading, with drive
wheels turning.
2. Check Park/Neutral (P/N) switch diagnosis chart.
3. If Park/Neutral (P/N) switch is OK, refer to Intermittents page.
CODE 25
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(HIGH TEMPERATURE INDICATED)
TEST DESCRIPTION:
The Manifold Air Temperature (MAT) sensor uses a thermistor to control the
signal voltage to the ECM The ECM applies a voltage (4-6 volts) on CKT 472
to the sensor. When manifold air is cold, the sensor (thermistor) resistance
is high, therefore the ECM will detect a high signal voltage. If the air
warms, the sensor resistance becomes less, and the voltage drops.
NOTE: A "Scan" tool may not be used to diagnose this fault, due to the ECM
transmitting "Default" (Substitute) values when the fault is present.
MAT Sensor
Temperature vs Resistance (Approx)
Deg. F Deg. C Ohms
210 100 185
160 70 450
100 38 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -18 25,000
-40 -40 100,700
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. Code 25 will
set if the engine is not experiencing turbocharger boost and the following
conditions are met:
Signal voltage indicates a manifold air temperature greater than 135'C
(275'F).
The above requirement is met for at least 30 seconds.
Due to the conditions necessary to set a Code 25, the "Check Engine" light
will only stay "ON" while the fault is present.
1. A "Scan" tool may not be used to diagnose this fault due to the ECM
transmitting "default" (substitute) values while the fault is present. If
voltage is above 4 volts, the ECM and wiring are OK.
2. If the resistance is less than 100 ohms, replace the sensor.
CODE 31
TURBO WASTEGATE OVERBOOST
CIRCUIT DESCRIPTION:
On turbo charged engines, the exhaust gases pass from the exhaust manifold
through the turbocharger, turning the turbine blades. The compressor side of
the turbocharger also turns, pulling air through the air filter and pushing
the air into the intake manifold, pressurizing the intake manifold.
The wastegate is normally closed, but opens to bypass exhaust gas to prevent an overboost condition. The wastegate will open when p ressure is a pp lied to the actuator, and is controlled by a wastegate control solenoid valve pulsed "ON" and" OFF" by the E CM. Under normal driving conditions, the control solenoid is energized all the time which closes "OFF" the manifold
pressure to the wastegate actuator. This allows for a rapid increase in
boost pressure. A boost increase will be detected by the MAP sensor, and the ECM will pulse the wastegate control valve. Manifold pressure will then be
allowed to pass to the wastegate actuator, and the actuator will open the
wastegate This will prevent an overboost condition on heavy acceleration. As
boost pressure decreases, the ECM closes the control valve and the wastegate actuator pressure bleeds "OFF" through the vent in the control valve. If an overboost does exist as indicated by the MAP sensor, the ECM will reduce fuel delivery to prevent damage to the engine.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. A Code 31 will set when the manifold absolute pressure exceeds 205 kPa of boost for two seconds, and a Code 33 has not previously been set. Code 31 will set, but the "Cheek Engine" light will stay "ON" only while the
overboost exists. The light will stay "ON" for 10 seconds after the
condition exists and then go "OUT."
An overboost condition could be caused by:
CKT 471 shorted to ground
A sticking wastegate actuator or wastegate
A control valve stuck in the closed position
A cut or pinched hose
A faulty ECM
An extremely dirty air filter
** With ignition shut "OFF," the control valve solenoid is open.
2. After the 103 kPa (15 psi) is applied to valve and then the pressure
source is removed, the actuator should slowly move back and close the
wastegate. If the pressure does not bleed "OFF," the vent in the control
valve solenoid could be plugged.
3. With the ignition "ON" and the diagnostic terminal grounded, the' control
valve solenoid should be energized. This closes "OFF" the manifold to the
wastegate actuator.
4. Check the electrical control portion of the system. With key "ON" and
engine not running, the solenoid should not be energized.
CODE 32
EXHAUST GAS RECERCULATION (EGR) CIRCUIT
CIRCUIT DESCRIPTION:
The ECM operates a vacuum solenoid to control the Exhaust Gas Recirculation
(EGR) valve. This solenoid is normally closed. By providing a ground path,
the ECM energizes the solenoid, allowing vacuum to reach the EGR valve.
Under certain conditions, when the EGR valve is normally open, the ECM tests
the EGR function by de-energizing the EGR control solenoid, blocking vacuum
to the EGR valve diaphragm. Without EGR, the system will sense a lean
condition and will increase the fuel integrator rate in response. The ECM
monitors the amount of fuel delivery increase. If the increase is below a
specified value, the ECM will interpret that the test was failed. The
failure indicates that closing the EGR valve when it would normally be open
does not make a significant change, indicating a problem in the EGR system.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart. The
diagnostic chart covers checks for the entire EGR system. If no trouble is
found but Code 32 was set, an intermittent electrical condition or a sticky
EGR valve is at fault.
DIAGNOSTIC AIDS:
The vacuum switch in the EGR solenoid assembly is not used.
An EGR valve stuck open will cause a rough idle.
A plugged EGR solenoid vent filter could cause the EGR valve to remain open
or to close slowly.
An inoperative check valve in the ported vacuum line will result in faulty
EGR system operation.
CODE 33
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT
(SIGNAL VOLTAGE HIGH - LOW VACUUM)
CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold
pressure (vacuum). The ECM receives this information as a signal voltage
that will vary from about l-l.5 volts at closed throttle idle, to 4-4.5
volts at wide open throttle (low vacuum or boost).
If the MAP sensor fails, the ECM will substitute a fixed MAP value and use
the Throttle Position Sensor (TPS) to control fuel delivery.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step will determine if Code 33 is the result of a hard failure or an
intermittent condition.
A Code 33 will set if:
MAP signal voltage is too high (low vacuum)
TPS less than 2%
These conditions for a time longer than 5 seconds
2. This step simulates conditions for a Code 34. If the ECM recognizes the
change, the ECM and CKTs 474 and 432 are OK. If CKT 452 is open, there may
also be a stored Code 23.
DIAGNOSTIC AIDS:
With the ignition "ON" and the engine stopped, the manifold pressure is
equal to atmospheric pressure and the signal voltage will be high. This
information is used by the ECM as an indication of vehicle altitude and is
referred to as BARO. Comparison of this BARO reading with a known good
vehicle with the same sensor is a good way to check accuracy of a "suspect"
sensor. Reading should be the same +-.4 volt.
A Code 33 will result if CKT 452 is open, or if CKT 432 is shorted to
voltage or to CKT 474. If Code 33 is intermittent, refer to Intermittents
page.
CODE 34
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR CIRCUIT
(SIGNAL VOLTAGE LOW - HIGH VACUUM)
CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold
pressure (vacuum). The ECM receives this information as a signal voltage
that will vary from less than 1.0 volt at closed throttle idle, to 4-4.5
volts at wide open throttle.
If the MAP sensor fails, the ECM will substitute a fixed MAP value and use
the Throttle Position Sensor (TPS) to control fuel delivery.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This step determines if Code 34 is the result of a hard failure or an
intermittent condition.
A Code 34 will set when:
MAP signal voltage is too low
Engine speed below 1200 rpm and/or TPS greater than 20%
2. Jumpering harness terminals "B" to "C", 5 volts to signal, will determine
if the sensor is at fault or if there is a problem with the ECM or wiring.
3. The "Scan" tool may not display battery voltage. The important thing is
that the ECM recognizes the voltage as more than 4 volts, indicating that
the ECM and CKT 432 are OK.
DIAGNOSTIC AIDS:
With the ignition "ON" and the engine stopped, the manifold pressure is
equal to atmospheric pressure and the signal voltage will be high. This
information is used by the ECM as an indication of vehicle altitude and is
referred to as BARO. Comparison of this BARO reading with a known good
vehicle with the same sensor is a good way to check accuracy of a "suspect"
sensor. Reading should be the same +-0.4 Volt.
A Code 34 will result if CKTs 474 or 432 are open or shorted to ground.
If CKT 416 is shorted to ground, there may also be a stored Code 22.
If Code 34 is intermittent, refer to Intermittents page.
CODE 35
IDLE SPEED ERROR
CIRCUIT DESCRIPTION:
The ECM controls engine idle speed with the IAC valve. To increase idle
speed, the ECM retracts the IAC valve pintle away from its seat, allowing
more air to bypass the throttle bore. To decrease idle speed, it extends the
IAC valve pintle towards its seat, reducing bypass air flow. A "Scan" tool
will read the ECM commands to the IAC valve in counts. Higher the counts
indicate more air bypass (higher idle). The lower the counts indicate less
air is allowed to bypass (lower idle). Code 35 will set when the closed
throttle engine speed is 225 rpm above or below the desired (commanded) idle
speed for 20 seconds.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. The Tech 1 rpm control mode is used to extend and retract the IAC valve.
The valve should move smoothly within the specified range. If the idle speed
is commanded (IAC extended) too low (below 700 rpm), the engine may stall.
This may be normal and would not indicate a problem. Retracting the IAC
beyond its controlled range (above 1500 rpm) will cause a delay before the
rpm's start dropping. This too is normal.
2. This test uses the Tech 1 to command the IAC controlled idle speed. The
ECM issues commands to obtain commended idle speed. The node lights each
should flash red and green to indicate a good circuit as the ECM issues
commands. While the sequence of color is not important if either light is
"OFF" or does not flash red and green, check the circuits for faults,
beginning with poor terminal contacts.
DIAGNOSTIC AIDS:
A slow, unstable, or fast idle may be caused by a non--IAC system problem
that cannot be overcome by the IAC valve. Out of control range IAC "Scan"
tool counts will be above 60 if idle is too low, and zero counts if idle is
too high. The following checks should be made to repair a non-IAC system
problem:
Vacuum Leak (High Idle)
If idle is too high, stop the engine. Fully extend (low) IAC with tester.
Start engine. If idle speed is above 800 rpm, locate and correct vacuum leak
including PCV system. Also check for binding of throttle blade or linkage.
System too lean (High Air/Fuel Ratio)
The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. Code 44 may be set. "Scan 02 voltage will be less than 300 mV (.3 volts). Check for low regulated fuel pressure, water in the fuel or a restricted injector.
System too rich (Low Air/Fuel Ratio)
The idle speed will be too low. "Scan" tool IAC Counts will usually be above
80. System is obviously rich and may exhibit black smoke in exhaust.
"Scan" tool O2 voltage will be fixed above 800 mV (.8 volt). Check for high
fuel pressure, leaking or sticking injector. Silicone contaminated O2 "Scan"
voltage will be slow to respond.
Throttle Body
Remove IAC valve and inspect bore for foreign material.
IAC Valve Electrical Connections
IAC valve connections should be carefully checked for proper contact.
PCV Valve
An incorrect or faulty PCV valve may result in an incorrect idle speed.
Refer to "Rough, Unstable, Incorrect Idle or Stalling" in "Symptoms," in
Section "B."
If intermittent poor driveability or idle symptoms are resolved by
disconnecting the IAC, carefully recheck connections, valve terminal
resistance, or replace IAC
CODE 42
ELECTRONIC SPARK TIMING (EST) CIRCUIT
CIRCUIT DESCRIPTION:
The ignition module sends a reference signal (CKT 430) to the ECM when the
engine is cranking. While the engine speed is under 400 rpm, the ignition
module will control ignition timing. When the engine speed exceeds 400 rpm,
the ECM applies 5 volts to the bypass line (CKT 424) to switch the timing to
ECM control (EST CKT 423).
When the system is running "ON" the ignition module, that is, no voltage on
the bypass line, the ignition module grounds the EST signal. The ECM expects
to see no voltage on the EST line during this condition. If it sees a
voltage, it sets Code 42 and will not go into the EST mode.
When the rpm for EST is reached (about 400 rpm), voltage will be applied to
the bypass line, the EST should no longer be grounded in the ignition
module, so the EST voltage should be varying.
If the bypass line is open or grounded, the ignition module will not switch
to EST mode, so the EST voltage will be low and Code 42 will be set.
If the EST line is grounded, the ignition module will switch to EST but,
because the line is grounded, there will be no EST signal. A Code 42 will be
set.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Confirms Code 42 and that the fault ca using the code is present.
2. Checks for a normal EST ground path through the ignition module. An EST
CKT 423 shorted to ground will also read less than 500 ohms, however, this
will be checked later.
3. As the test light voltage touches terminal "C7," the module should
switch, causing the ohmmeter to "overrange" if the meter is in the 1000~2000 ohms position. Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
4. The module did not switch and this step checks for:
EST CKT 423 shorted to ground
Bypass CKT 424 open
Faulty ignition module connection or module
5. Confirms that Code 42 is a faulty ECM and not an intermittent in CKTs 423
or 424.
CODE 43
ELECETRONIC SPARK CONTROL (ESC) CIRCUIT
CIRCUIT DESCRIPTION:
The knock sensor is used to detect engine detonation and the ECM will retard
the electronic spark timing based on the signal being received. The
circuitry within the knock sensor causes the ECM 5 volts to be pulled down
so that under a no knock condition, CKT 496 would measure about 2.5 volts.
The knock sensor produces an A/C signal which rides on the 2.5 volts, DC
voltage. The amplitude and signal frequency is dependent upon the knock
level.
If CKT 496 becomes open or shorted to ground the voltage will either go
above 3.5 volts or below 1.5 volts. If either of these conditions are met
for about 5 seconds, a Code 43 will be stored.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 43 will set when:
Coolant temperature is over 90'C
MAT temperature is over O'C
High engine load based on MAP and rpm
Voltage on CKT 496 goes above 3.5 volts or below 1.5 volts
All conditions present for 5 seconds
If an audible knock is heard from the engine, repair the internal engine
problem, as normally no knock should be detected at idle.
2. If tapping on the engine lift hook does not produce a knock signal, try
tapping engine closer to sensor before proceeding.
3. The ECM has a 5 volts pull-up resistor, which should be present at the
knock sensor terminal.
4. This test determines if the knock sensor is faulty or if the ESC portion
of the Mem-Cal is faulty.
DIAGNOSTIC AIDS:
Check CKT 496 for a potential open or short to ground. Also check for proper
installation of Mem-Cal
Refer to the Intermittents page.
CODE 44
OXYGEN (O2) SENSOR CIRCUIT
(LEAN EXHAUST INDICATED)
CIRCUIT DESCRIPTION:
The ECM applies a bias voltage of approximately 450 millivolts (350-550 mV
is normal bias voltage) between terminals "GEI4" and "GElS." The oxygen (02)
sensor varies the voltage within a range of about 1 volt, if the exhaust is
rich, down through about .10 volt, if exhaust is lean. As each pair of
exhaust valves open, a different exhaust oxygen content passes by the oxygen sensor, which causes the oxygen sensor voltage output to vary. A lean exhaust condition will cause the oxygen sensor to output a low voltage,
which will pull the bias voltage from CKT412 low. The ECM is programmed to
interpret any voltage less than 500 mV as a "lean exhaust condition."
The sensor is like an open circuit and produces no voltage when it is below
about 360'C (680'F). An open sensor circuit causes "Open Loop" operation.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. This test checks the oxygen sensor's heating element. The heating element
resistance should be 3.5 ohms at 20'C (68'F) or 14 ohms at 350'C (562'F).
2. Code 44 is set when the oxygen (02) sensor signal voltage on CKT 412;
Remains below .2 volt for 2 minutes and the fuel system is operating in
"Closed Loop."
DIAGNOSTIC AIDS:
Using the Tech I diagnostic computer "Scan" tool, observe the block learn
values at different rpm and air flow conditions. The "Scan" tool also
displays the block cells, so the block learn values can be checked in each
of the cells to determine when the Code 44 may have been set. If the
conditions for Code 44 or Code 64 exist, the block learn values will be
around 150.
O2 Sensor Wire. Sensor pigtail may be mispositioned and contacting the
exhaust manifold.
Check for intermittent ground in wire between connector and sensor.
Poor connection at oxygen (02) sensor ground wire.
Lean Injector(s).
Fuel Contamination. Water, even in small amounts, near the in-tank fuel
pump inlet can be delivered to the injectors. The water causes a lean
exhaust and can set a Code 44 or Code 64.
Fuel Pressure. System will be lean if pressure is too low. It may be
necessary to monitor fuel pressure while driving the car at various road
speeds and/or loads to confirm.
Exhaust Leaks. If there is an exhaust leak, the engine can cause outside
air to be pulled into the exhaust and past the sensor. Vacuum or crankcase
leaks can cause a lean condition.
If the above are OK, it is a faulty oxygen sensor.
CODE 45
OXYGEN (O2) SENSOR CIRCUIT
(RICH EXHAUST INDICATED)
CIRCUIT DESCRIPTION:
The ECM supplies a voltage of about .45 volt between terminals "GE 14" and
"GE 15". (If measured with a 10 megohm digital voltmeter, this may read as
low as .32 volt.) The O2 sensor varies the voltage within a range of about 1
volt if the exhaust is rich, down through about .10 volt if exhaust is lean.
The sensor is like an open circuit and produces no voltage when it is below
about 316'C (600'F). An open sensor circuit causes "Open Loop" operation.
The heating element in the O2 sensor causes the sensor to heat up quickly,
allowing for quicker closed-loop operation.
TEST DESCRIPTION:
Numbers below refer to circled numbers on the diagnostic chart.
1. Code 45 is set when the O2 sensor voltage:
Remains above .75 volt for 50 seconds; and the system is in "Closed Loop."
DIAGNOSTIC AIDS:
Using the "Scan," observe the block learn values at different rpm and air
flow conditions. The "Scan" also displays the block cells, so the block
learn values can be checked in each of the ceHs to determine when the Code
45 may have been set. If the conditions for Code 45 exists, the block learn
values witor)l be around 115.
Fuel Pressure. System will go rich if pressure is too high. The ECM can
compensate for some increase. However, if it gets too high, a Code 45 may be set.
Rich Injector. Perform injector balance test.
Leaking Injector.
Check for fuel contaminated oil.
HEI Shielding. An open ignition ground CKT 453 may result in EMI, or
induced electrical "noise." The ECM looks at this "noise" as reference
pulses. The additional pulses result in a higher than actual engine speed
signal. The ECM then delivers too much fuel, causing system to go rich.
Engine tachometer will also show higher than actual engine speed, which can
help in diagnosing this problem.
Canister purge. Check canister for fuel saturation. If full of fuel, check
canister control and hoses. See "Purge Valve Operation" Section "C3."
MAP Sensor. An output that causes the ECM to sense a higher than normal
manifold pressure can cause the system to go rich. Disconnecting the MAP
sensor will allow the ECM to set a fixed value for the sensor. Substitute a
different MAP sensor if the rich condition is gone while the sensor is
disconnected.
Check for leaking fuel pressure regulator diaphragm by checking vacuum line
to regulator for fuel.
TPS. An intermittent TPS output will cause the system to go rich, due to a
false indication of the engine accelerating.
EGR. An EGR staying open (especially at idle) will cause the O2 sensor to
indicate a rich exhaust, and this could result in a Code 45
CODE 51
PROM ERROR
(FAULTY OR INCORRECT PROM)
TEST DESCRIPTION:
Check that all pins are fully inserted in the socket and that PROM is
properly seated. If OK, replace PROM, clear memory, and recheck.
If Code 51 reappears, replace ECM.