OBD I Code Retrieval and Typical Code List


HotZ28
03-01-2007, 09:58 PM
If you have an OBD I GM vehicle with the 12-pin ALDL, all you need is a simple paper clip to read the codes. Locate the ALDL (Assembly Line Diagnostic Lead). It is located under the driver's side dash. Insert paper clip between terminals A & B as shown below.
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Turn key on, but do not start engine. The check engine light will begin to flash. The ECM (computer) will display a "code 12" by flashing the "service engine soon" light, indicating that the system is operating. A code 12 is simply one flash, followed by a brief pause, then two flashes in quick succession. This code will be flashed 3-times. If no other codes are stored, code 12 will flash until the diagnostic terminal ground is removed. After code 12 has flashed 3-times, the ECM will display any stored trouble codes. Each code will be flashed 3-times, then code 12 will be flashed again, indicating that the display of any stored trouble codes has been completed.


Code 12
1 flash, pause, 2 flashes
Diagnostic: This Code will flash whenever the diagnostic terminal is grounded with the ignition switch turned on and the engine is not running.

Code 13
1 flash, pause 3 flashes
Oxygen sensor voltage stays between 0.35 and 0.55 volts for 30 seconds when the TPS signal was above 0.55 volts. The engine had been running for at least 40 seconds with an engine temperature above 110 F.

Code 14
1 flash, pause, 4 flashes
Coolant temperature sensor signal indicates a temperature of over 284 F for 4 seconds after the engine had been running for at least 10 secondsCoolant Sensor Circuit (high temperature) If the engine is experiencing overheating problems, have those fixed before continuing. Check all wiring and connectors associated with the coolant temperature sensor. Replace the Coolant temperature sensor.

Code 15
1 flash, pause, 5 flashes
Coolant Sensor Circuit (low temperature) Coolant temperature sensor signal indicates a temperature colder than -36 F for at least 4 seconds after the engine had been running for 2 seconds. See above, then check wiring connections at the ECM

Code 16
1 flash, pause, 6 flashes
Charging system voltage was either below 9 volts or above 17 volts for 10 seconds. System High Voltage If the voltage is above 17.1 volts for more than 2 seconds, the Service Engine Soon light will illuminate. Have the charging system checked.

Code 17
1 flash, pause, 7 flashes
PCM did not detect any spark reference pulses within 240 fuel control reference pulses (80 crank revolutions) while engine was running below 1200 RPM. Spark Reference Circuit.

Code 21
2 flashes, pause, 1 flash
Throttle Position Sensor, TPS voltage was above 0.8 volts for 5 seconds when engine was running and air flow was less than 15 gm/sec, or the TPS voltage was over 4.8 volts at any time.

Code 22
2 flashes, pause, 2 flashes
Throttle Position Sensor check TPS adjustment. Replace the TPS sensor. TPS voltage was under 0.2 volts for 4 seconds when the ignition was on. Check for a sticking, or mis-adjusted throttle position sensor.

Code 23
2 flashes, pause, 3 flashes
Inlet air temperature sensor signal showed an air temperature of -29 F for 4 seconds. Manifold Air Temperature. Check the MAT sensor, wiring, and connectors for a possible open circuit. replace the MAT sensor.

Code 24
2 flashes, pause, 4 flashes
Vehicle speed sensor signal showed less than 3 MPH for 2 seconds when engine speed was greater than 3000 RPM and transmission has not been in park or neutral for more than 4 seconds. Or vehicle speed goes from above 18 MPH to 0 in less than 2 seconds without applying brake. This code can only appear when the vehicle is in motion, if it appears when the car is not in motion, ignore it. Check ECM connections, any other problems should be diagnosed by a qualified mechanic.

Code 25
2 flashes, pause, 5 flashes
Inlet air temperature sensor signal showed an air temperature of over 218 F for 5 seconds when vehicle speed was greater than 35 MPH. Check the voltage between the MAT sensor and the ECM. voltage should be above 4 volts.

Code 26
2 flashes, pause, 6 flashes
Quad-Driver sense voltage shows a low voltage when battery voltage should be present, or if the sense voltage shows battery voltage when a low voltage should be present for at least 5 seconds when the engine was running. Have the car checked by a dealer.

Code 27
2 flashes, pause, 7 flashes
2nd gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 2nd or 4th gear switch was open when engine was first started. Gear switch Diagnosis Have the car checked by a dealer.

Code 28
2 flashes, pause, 8 flashes
3rd gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 3rd or 4th gear switch was open when engine was first started.

Code 29
2 flashes, pause, 9 flashes
4th gear switch was closed or grounded for 10 seconds when vehicle was in 4th gear, or 4th gear switch was open when engine was first started.

Code 31
3 flashes, pause, 1 flash
PRNDL indicates a gear other than park or neutral for 13 seconds for 3 consecutive startups when ignition was on, engine was off and vehicle speed was 0. Or the PRNDL status indicators (A, B, C, P) showed an incorrect combination of Lo and Hi for more than 10 seconds.
Check all connectors, and for a faulty switch.

Code 32
3 flashes, pause 2 flashes
EGR System Check vacuum hoses and connections for leaks and restrictions. Replace the EGR solenoid or valve.

Code 33
3 flashes, pause, 3 flashes
MAF Sensor Have the mass air flow sensor replaced.

Code 34
3 flashes, pause, 4 flashes
There was no MAF signal for over 4 seconds while engine was running. Check for loose or damaged air duct, misadjusted minimum idle speed and cacuum leaks. Inspect the MAF sensor and the electrical connections.

Code 38
3 flashes, pause, 8 flashes
The Torque Converter Clutch portion of the brake switch was open and caused the TCC to be inoperative, adjust OR replace switch.

Code 39
3 flashes, pause, 9 flashes
Torque Converter Clutch Check ECM connections, check for faulty Torque Conver Clutch solenoid.

Code 41
3 flashes, pause, 1 flash
Cam sensor signal was not received by PCM for the last 5 seconds while engine was running. Check for faulty connections, or a faulty cam sensor. Maybe also indicate a problem with the vehicle's timing chain.

Code 42
4 flashes, pause, 2 flashes
The Electronic Spark Timing (EST) signal did not change when the ECM applied bypass voltage to the ignition module. Check for faulty connections, or a faulty ignition module.

Code 43
4 flashes, pause, 3 flashes
Knock sensor signal was either above 3.5 volts or below 1.5 volts for at least 20 seconds. Check for a faulty knock sensor, or Mem-Cal.

Code 44
4 flashes, pause, 4 flashes
Oxygen sensor voltage was under 0.25 volts for up to 4 1/2 minutes of closed loop operation, lean exhaust. Check the ECM wiring connections. Check for vacuum leaks at the hoses and intake manifold gasket.

Code 45
4 flashes, pause, 5 flashes
Oxygen sensor voltage was over 0.75 volts for 2 minutes in closed loop operation with a throttle angle between 0.6 and 1.9 volts. Rich exhaust. Check the evaporative charcoal canister and its components for the presence of fuel.

Code 46
4 flashes, pause, 6 flashes
Power Steering pressure switch Check for faulty connections, or a faulty switch.

Code 48
4 flashes, pause 8 flashes
Misfire Diagnosis. Check the ignition and fuel systems.

Code 51
5 flashes, pause, 1 flash
Prom or Mem-Cal Make sure the MEM-Cal or prom is properly installed in the ECM. Replace Mem-Cal or prom.

Code 52
5 flashes, pause, 2 flashes
Cal-Pak Check the Cal-Pak to insure properation. replace the Cal-Pak.


Code 54
5 flashes, pause, 4 flashes
EGR #2 solenoid was out of range for 6 of 8 tests. 55 EGR #3 solenoid was out of range for 3 of 5 tests.

Code 55
5 flashes, pause, 5 flashes
ECM Check for faulty connections. Replace the ECM.

Code 56
5 flashes, pause, 6 flashes
Quad-driver voltage was incorrect for 5 seconds on the circuit connected to QDM "B".

Code 57
5 flashes, pause, 7 flashes
Boost control solenoid did not open bypass valve causing engine torque to be too high.

Code 63
6 flashes, pause, 3 flashes
EGR flow was incorrect. EGR solenoid did not work properly. Have vehicle serviced by a dealer.

Code 64
6 flashes, pause, 4 flashes
Same as Code 63 EGR flow was incorrect. EGR solenoid did not work properly.

Code 65
6 flashes, pause, 5 flashes
Same as Code 63.

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