Apex Automotive and Emissions ASE Certification

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GM Truck OBD 1 Codes and Instructions


Checking trouble codes on the OBD-I system used in GM trucks from 1988 to 1995 is easy. It requires no tools other than a paper clip or small piece of wire for jumping the A & B terminals on the ALDL connector.   The ALDL connector is under the driver's side of the dash. Here's a picture showing the A & B terminals at the top right:








1.  With the ignition switch off,  jump the A & B terminals on the ALDL connector.

2.  Turn the ignition switch on (but do not start engine)

3.  Watching the "Service Engine Soon" light on the dash.
4. Each code will be represented by the SES light flashing, pausing, and flashing.
5. Normal operation begins with code 12, represented by a flash, pause, flash flash indicating "1" and "2" meaning "12".
6. Each code will appear 3 times before moving on to the next. Codes are display in numerical order.
7. When the list of codes has completed, the list will repeat, starting again with 12.
8. Code 12 is normal and not an indicator of a problem.

Please note that a code is a starting point and does not necessarily mean the particular sensor or system is bad. Codes can often be caused by other problems and the particular sensor is only reporting that its set of data is out of range; it is up to the troubleshooter to determine the cause of the problem.

Codes can be cleared by disconnecting the battery for 20 seconds. Codes will also clear if after 50 starts the problem has not reoccurred.

This is a list of codes relating to gasoline engines.  Not all codes apply to all systems. If you have a code not listed, contact Apex Automotive and we'll further research for you.

13 - O2 (oxygen sensor) circuit open
14 - ECT/CTS (engine coolant temperature) sensor circuit low (high temperature)
15 - ECT/CTS (engine coolant temperature) sensor circuit high (low temperature)
16 - VSS (vehicle speed sensor) buffer fault
21 - TPS (throttle position sensor) circuit high (signal voltage high)
22 - TPS (throttle position sensor) circuit low (signal voltage low)
24 - VSS (vehicle speed sensor) circuit low
28 - TR (transmission range) pressure switch assembly
32 - EGR (exhaust gas recirculation) valve error
33 - MAP (manifold absolute pressure) sensor circuit high (low vacuum)
34 - MAP (manifold absolute pressure) sensor circuit low (high vacuum)
35 - IAC (idle air control) error
36 - Idle speed control actuator error (when equipped)
42 - IC (ignition control) error
43 - KS (knock sensor) error
44 - Lean exhaust
45 - Rich exhaust
51 - PROM (memcal, chip) error
53 - System voltage high (supply to ECM/PCM)
54 - Fuel pump relay circuit low voltage
55 - ECM/PCM error