Reading and Clearing Diagnostic Trouble Codes

ALDL Connector:

PROCEDURE:

Turn the ignition to the "ON" or "RUN" position (engine off). Make a note of the "SERVICE ENGINE SOON" light. Light should come on steady. If there is no light or the light flashes, do not proceed with remainder of test. Results of test will be invalid.
Turn the key on to the "RUN" position (engine off). Ground the diagnostic terminal (ALDL terminal A to B). The "SERVICE ENGINE SOON" ("CHECK ENGINE") light should flash a code 12. This is displayed, on the Check Engine light, by 1 flash followed by a short pause then 2 flashes. This should repeat 3 times. A code 12 should always flash when an on-board diagnostic check is performed. Any other codes, stored in the computer (PCM), will follow a code 12.