GM trouble codes may be retrieved with or without a code scanner on pre-1993 GM vehicles, due to the type of OBD port connector. This port has been changed in the later model vehicles, and a scan tool is necessary to retrieve the codes. Once the codes have been retrieved, the accompanying site will have a list of the codes and their meaning. To erase the codes and turn the check engine light out, the problem associated with the code must be repaired.
Locate the OBD (on-board diagnostic) terminal below the dash and just to the left of the steering column. If it is rectangular and has 12 cavities, it is the old style, and codes can be retrieved without a scanner. If it is square with pins, it is the new style.
Place the paper clip in the far top-right terminals. Turn the ignition key to the "on" position, and make note of the check engine light flashes. Count the number of flashes in sequence between pauses. Each code consists of two numbers. The number 12 will always flash first. It just means the computer is not receiving any signals indicating the vehicle is running. The engine is off, so it will repeat this twice before preceding. A "flash, flash, flash, pause, flash" is interpreted as a code 31. It will flash each code three times and move on to the next code.
Remove the paper clip when all the codes are retrieved and turn off the ignition. Cross-reference the codes with the link below. It has a listing of all the early codes and their meanings.
Repair all code problems in order to reset the computer and turn off the light. To reset the computer, disconnect the negative battery cable for 5 minutes and re-connect the terminal.
Connect the code scanner to the OBD port to the left of the steering column and below the dash. Turn the key on without starting the vehicle. Press the "Read" key on the scanner. Make a note of any codes. Turn the key off.
Cross-reference the codes with the definitions on the site below. Repair all faults before attempting to reset the computer.
Turn the key on and press the "Erase" key on the code scanner.
