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RETRIEVING TROUBLE CODES GMC Sonoma I

Diagnosis - A/t 1 illustration ~352 words

RETRIEVING TROUBLE CODES

The ECM stores component failure information under a related trouble code, which can be recalled later for diagnosis and repair. When recalled, these codes can be displayed on a scan tester, or by entering diagnostic mode and observing flashes of the SERVICE ENGINE SOON light. It is NOT necessary to enter diagnostic mode to use TCC test charts; however, it will be necessary to enter ECM diagnostic mode to verify the presence of any codes.

Note. Even when using a scan tester, it is recommended that the diagnostic mode be accessed in advance to verify the computer's ability to self-diagnose.

Scheme 55

Scheme 55: ENTERING DIAGNOSTIC MODE
Scheme 55: ENTERING DIAGNOSTIC MODE
  1. To enter diagnostic mode, turn ignition switch on, but do not start engine. SERVICE ENGINE SOON light should glow. Locate Assembly Line Diagnostic Link (ALDL) connector attached to ECM wiring harness under instrument panel. Insert jumper wire across diagnostic mode "test" terminal "B" and ground terminal "A". (Scheme 55) CAUTION: Inserting jumper wire in terminals of ALDL connector completes test circuit. DO NOT ground ALDL connector until after ignition switch is on. Inserting jumper wire in terminals of ALDL connector with engine running will cause fuel injected vehicles to enter field service mode. Flashes of the SERVICE ENGINE SOON light will not indicate codes if this is done.
  2. SERVICE ENGINE SOON light should flash Code 12 (FLASH, pause, FLASH, FLASH) followed by a longer pause. Trouble Code 12 will be repeated 3 more times, then if any trouble codes are stored in the ECM memory, they will be displayed in the same manner.
  3. Trouble codes will be displayed from lowest to highest numbered codes (3 times each) and be repeated as long as the test terminal of the ALDL connector is grounded.
  4. To exit diagnostic mode, remove jumper wire from ALDL connector and turn ignition switch off.

Note. If Code 12 is not flashed when the ALDL diagnostic test terminal is grounded, information received by a scan tester on the serial data line may not be accurate. For complete testing of the ECM diagnostic circuit, refer to appropriate article in the ENGINE PERFORMANCE section.