The GM vehicles that cannot be tested using this procedure are Cadillac,Geo,Nova and Sprint.

Items needed;Code Key,paper clip,small double blade fuse,paper and pencil.

Step 1) To access the on-board computer trouble codes,locate the Assembly Line Diagnostic Link (ALDL).The ALDL on most GM vehicles is located under the dashboard on the drivers side.

Step 2) Take the fuse or paper clip(bent in a u shape) and insert it into the A and B ports of the ALDL terminal.

Step 3) Turn the vehicles ignition on, but DO NOT START THE ENGINE. 

Step 4) Watch for the CHECK ENGINE LIGHT to flash Code 12,that Code 12 shows you have correctly accessed the computer.The code will flash this way; one flash for the 1 digit, a pause,then two flashes for the 2 digit.Code 12 will flash three times.If there is no problem Code 12 will continue to flash.

Step 5) If there is a problem, one or more trouble codes will flash.Watch for the codes to flash in the same manner as Code 12.Example:Code 44 is: four flashes,a pause,four flashes 

Step 6) After you have read the trouble codes once,wait for the cycle to begin again.This time, write down the codes as they flash 

Step 7) Once you have obtained the trouble codes,turn off the ignition switch.Remove the code key from the ALDL.

Step 8) Match your vehicles trouble codes from the list below.

The computer-controlled part failure that each trouble code indicates.

Here are the most common trouble codes.

Code 12-diagnostic mode              Code 13-oxygen sensor or circuit

Code 14-coolant sensor or circuit/high temperature indicated

Code 15- coolant sensor or circuit/low temperature indicated

Code 21-throttle position sensor circuit or plunger

Code 22-throttle position sensor out of adjustment

Code 23-manifold air temperature sensor

Code 33-manifold absolute pressure sensor or circuit (low vacuum)

Code 33-mass air flow sensor or circuit         Code 34-mass air flow sensor or circuit

Code 35-idle speed control switch or circuit (shorted)

Code 44-oxygen sensor or circuit-lean exhaust

Code 45-oxygen sensor or circuit-rich exhaust

Code 55-oxygen sensor circuit or electronic control module (ECM)

Code 61-oxygen sensor signal faulty

There may be other reasons for trouble codes appearing.If you test the parts indicated and they test good,then you need to consult the shop manuel or a repair facility.