C3 Self Diagnostics

ALDL Connector:

Use a "Scan" tool or ground the diagnostic test terminal with the engine not running and the ignition switch in the "ON" position in order to retrieve codes. The "SERVICE ENGINE SOON" light will flash a Code 12 three times and then flash each code stored in memory three times, if the "Scan" tool is not used. Once all Codes are retrieved, a Code 12 will follow. New codes cannot be stored in the ECM once the diagnostics mode is activated. See DIAGNOSTIC CIRCUIT CHECK for a complete system check.