Diagnostic Trouble Codes
From GM Wiki
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Retrieving the Diagnostic Trouble Codes
To extract the DTCs from ECM memory you need to use a short jumper on terminals A & B on the ALDL connector. This terminal is part of an electrical connector located just behind the dashboard, next to the steering column. A small rectangular ploate is used to cover the connector and must be pried out of place to provide access to the terminals. With the connector exposed to view, push one end of the jumper wire into the TEST terminal and the other end into the GROUND terminal. Caution: Do not start the engine with the TEST terminal grounded.
Turn the ignition into the on position (engine not running). The "CHECK ENGINE" (CEL) or "SERVICE ENGINE SOON" (SES) light should flash trouble code 12, indicating that the diagnostic system is working. Code 12 will consist of one flash, followed by a short pause, and then two flashes in quick succession. After a longer pause, the code will repeat itself two more times.
If no other codes have been stored, Code 12 will continue to repeat itself until the jumper wire is disconnected. If additional trouble codes have been stored, they will follow code 12. Again, each trouble code will flash three times before moving on.
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Trouble Code Identification
Following is a list of the typical trouble codes which may be encountered while diagnosing the Electronic Control Module system. Also included are simple troubleshooting procedures. Component replacement, where suggested, may not cure the problem in all cases. For this reason you may want to seek professional advice before purchasing replacement parts. To clear trouble codes from the ECM memory, disconnect the negative battery cable. Disconnecting the power to the ECM can be an important diagnostic tool, especially on intermittent problems. O
Trouble Code |
Circuit or System |
Probable Cause |
|
12 |
(one flash, pause, two flashes) |
No reference pulses to ECM |
This code should flash whenever the Test terminal is grounded, with the ignition on, and the engine not running. If additional Trouble codes are stored (indicating a problem), they will appear after this code has flashed three times. With the engine running, the appearance of this code indicates that no references from the distributor are reaching the ECM. Carefully check the four-terminal EST connector at the distributor. |
13 |
(one flash, pause, 3 flashes) |
Oxygen sensor circuit |
Check the wiring and connections from the oxygen sensor. Replace the oxygen sensor. |
14 |
(one flash, pause, four flashes) |
Coolant sensor circuit |
If the engine is experiencing overheating problems, the problem must be rectified before continuing. Check all wiring and connectores associated with the sensor. Replace the coolant sensor. |
15 |
(one flash, pause, five flashes) |
Coolant sensor circuit (low temperature indicated) |
See above. Also, check the thermostat for proper operation. |
19 |
(one flash, pause, nine flashes) |
Crankshaft Position Sensor (CPS) reference signal intermittent |
The reference signal from the CPS is intermittent. Inspect the PCM harness connector and crankshaft sensor wire harness for shorts or a damaged harness. |
21 |
(two flashes, pause, one flash) |
TPS circuit (signal voltage high) |
Check for sticking or misadjusted TPS. Check all wiring and connections at the TPS and the ECM. Adjust or replace TPS |
22 |
(two flashes, pause, two flashes) |
TPS circuit (signal voltage low) |
See above. |
23 |
(two flashes, pause, three flashes) |
Mixture Control (M/C) solenoid circuit (carb'ed models only) |
Check the electrical connections at the M/C solenoid. If OK, clear the Ecm memory and recheck for code(s) after driving the vehicle. Check wiring connections at the ECM. Check wiring from M/C solenoid. |
23 |
(two flashes, pause, three flashes) (1986 through 1990 models) |
Manifold Air Temperature (MAT) sensor circuit (low temperature) |
Check the MAT sensor, wiring, and connections for an open circuit. Replace the MAT sensor. |
23 |
(two flashes, pause, three flashes) (1990 and later models) |
Intake Air Temperature (IAT) sensor circuit (low temperature) |
See above. |
24 |
(two flashes, pause, four flashes) |
Vehicle Speed sensor (VSS) circuit |
A fault in this circuit should be indicated only while the vehicle is in motion. Disregard code 24 if set when drive wheels are not turning. Check connection at the ECM. Check the TPS setting. |
25 |
(two flashes, pause, five flashes) (1990 and earlier models) |
Manifold Air Temperature (MAT) sensor circuit (high temperature) |
Check the resistance of the MAT sensor. Check the wiring and connections to the sensor. Replace the MAT sensor. |
25 |
(two flashes, pause, five flashes) (1991 and later models) |
Intake Air Temperature (IAT) sensor circuit (high temperature) |
Check the resistance of the IAT sensor. Check the wiring and connections to the sensor. Replace the IAT sensor. |
26 |
(two flashes, pause, six flashes) (1994 2.2L models) |
Quad-Driver Module (QDM) circuit |
QDMs are switches within the PCM used to control various components such as, canister purge valve, EGR solenoid, cooling fan relay, etc. Due to the complexity of they system, it is suggested that diagnosis and repair of QDM related malfuctions be left to an authorized repair facility. Replace the PCM. |
27/28 |
(two flashes, pause, seven or eight flashes) (1994 2.2L models) |
QDM Circuits |
See above. |
31 |
(three flashes, pause, one flash) |
Wastegate actuator (turbocharged models only) |
Possible sticking wastegate actuator or wastegate. Also power to the ignition and/or faulty ECM. |
32 |
(three flashes, pause, two flashes) |
No altitude compensator voltage (carb'ed models only) |
Check connections at altitude compensator. Check for an open circuit in the wiring from the sensor to the ECM. Replace the altitude compensator. |
32 |
(three flashes, pause, two flashes) |
Exhaust Gas Recirculation (EGR) system failure |
Check the vacuum source and all vacuum lines. Check the electrical connectors at the ECM and EGR valve. Replace the EGR valve or ECM as necessary. |
33 |
(three flashes, pause, three flashes) |
Manifold Absolute Pressure (MAP) sensor or circuit |
Check vacuum hose(s) from MAP sensor. Check electrical connections at the ECM. Replace the MAP sensor. |
33 |
(three flashes, pause, three flashes) |
Mass Air Flow (MAF) sensor or circuit (excessive air flow) |
Excessive air flow indicated. Check terminal C on the MAF sensor: it should be about .5 volts at idle and 4.7 volts at wide open throttle. Trace the wire from terminal C and look for an open circuit condition. Replace the MAF. |
34 |
(three flashes, pause, four flashes) |
Vacuum sensor circuit (carb'ed models only) |
Check the wiring leading to terminals 20, 21, and 22 of the ECM. Check the connections at the ECM. Check the vacuum sensor, wiring, and connectors. Replace the Vacuum sensor. |
34 |
(three flashes, pause, four flashes) |
Mass Air Flow (MAF) sensor or circuit (low air flow) |
Low air flow indicated. Check terminal C on the MAF sensor: it should be about .5 volts at idle and 4.7 volts at wide open throttle. Trace the wire from terminal C and look for an short to ground condition. Replace the MAF. |
34 |
(three flashes, pause, four flashes) |
Manifold Absolute Pressure (MAP) sensor or circuit |
Check for an open or shorted to ground circuit. Check the MAP sensor. Replace the MAP sensor if it is malfunctioning. |
35 |
(three flashes, pause, five flashes) |
Idle Air Control (IAC) valve circuit |
Idle RPM too low or too high. Check minimum idle speed. Check fuel pressure, check for leaking injectors and obstructions in the throttle body. Replace the IAC valve. |
41 |
(four flashes, pause, one flash) |
No distributor reference signal to the ECM at specified vacuum (carb'ed models only) |
Check all wires and connections at the distributor. Check distributor pickup coil connections. Check vacuum sensor circuit. (see above) |
41 |
(four flashes, pause, one flash) |
Cylinder select error |
Remove access cover on the ECM and check to see that the MEM-CAL, PROM, or CAL-PAK is installed properly. Clear the trouble code and see if the code resets. If so, replace the MEM-CAL, PROM, or CAL-PAK. |
42 |
(four flashes, pause, two flashes) |
Electronic Spark Timing (EST) circuit |
Check the wiring and connections between the ignition module, and the ECM. Replace the ignition module. Replace the ECM. |
43 |
(four flashes, pause, three flashes) |
Electronic Spark Control (ESC) circuit or Knock Sensor (KS) circuit |
Check wiring and connections from the Knock sensor to the ESC controller or the ECM for an open circuit or short to ground. If necessary, reroute the harness away from other wires such as spark plugs, etc. Replace the Knock sensor. |
44 |
(four flashes, pause, four flashes) |
Lean Exhaust |
Check the wiring and connectors from the oxygen sensor to the ECM. Check the ECM ground terminal. Check for a sticking M/C solenoid. Check the fuel pressure. Check for vacuum leaks at the carb base gasket, throttle body gasket, vacuum hoses or intake manifold gasket. Replace the oxygen sensor. |
45 |
(four flashes, pause, five flashes) |
Rich Exhaust |
Check for a sticking M/C solenoid. Check wiring at M/C solenoid connector. Check the evaporative charcoal canister and its components for the presence of fuel. Check for fuel contaminated oil. Check the fuel pressure regulator. Check for a leaking fuel injector. Check for a sticking EGR valve. Replace the oxygen sensor. |
51 |
(five flashes, pause, one flash) |
PROM/EEPROM error |
Faulty or incorrect prom/EEPROM. Diagnosis should be performed by a dealer service department or other repair shop. Replace the ECM. |
52 |
(five flashes, pause, two flashes) |
CAL-PAK error |
Faulty or incorrect CAL-PAK. Diagnosis should be performed by a dealer service department or other repair shop. Replace the ECM |
53 |
(five flashes, pause, three flashes) |
System over-voltage |
Code 53 will set if the voltage at the ECM is greater than 17.1 volts or less than 10 volts. Check the charging system. |
54 |
(five flashes, pause, four flashes) |
Mixture Control (M/C) solenoid circuit (carb'ed models only) |
Check all M/C solenoid, ECM wires, and connections. Replace the M/C solenoid. |
54 |
(five flashes, pause, four flashes) |
Fuel pump circuit |
Check the fuel pump relay, circuit, and connections. Check the oil pressure switch. Repair/replace faulty components. |
55 |
(five flashes, pause, five flashes) |
ECM/PCM |
Be sure the ECM/PCM ground connections are tight. If they are, replace the ECM/PCM |
61 |
(six flashes, pause, one flash) |
Contaminated Oxygen sensor |
Replace the oxygen sensor. A contaminated sensor can be caused by fuel additives that contain silicon, or non-GM approved sealants/lubricants. |
62 |
(six flashes, pause, two flashes) |
Transaxle gear switch signal circuit |
Check for an open circuit with a voltmeter at the TCC connector. Voltage should be about 12 volts. Further diagnosis is best left to a dealer service shop, or other repair facility with the proper diagnostic tools. |
63 |
(six flashes, pause, three flashes) |
Manifold Absolute Pressure (MAP) sensor circuit (low vacuum detected) |
Check wiring and connections from the MAP sensor. To the ECM. Check the vacuum hose to the sensor for leaks. Replace the MAP sensor. Replace the ECM. |
64 |
(six flashes, pause, four flashes) |
Manifold Absolute Pressure (MAP) sensor circuit (high vacuum detected) |
Check the wiring and connections from the MAP sensor to the ECM. Replace the MAP sensor.Replace the ECM |
66 |
(six flashes, pause, six flashes) |
A/C refridgerant pressure sensor circuit |
Check the sensor electrical terminal connections and for a possible short to ground or open circuit in the sensor wiring. Replace the A/C refrigerant pressure sensor. |
