Diagnostic Procedure

Diagnosis of the computerized engine control system should be performed in the following order:

1. Make sure all engine systems not related to the computer system are operating properly. Do not proceed with testing unless all other problems have been repaired. Appropriate DIAGNOSTIC CIRCUIT CHECK for that system MUST be performed prior to utilizing trouble code charts. See BASIC TESTING article.
2. If trouble codes were displayed (other than Code 12), decide whether codes are hard or intermittent trouble codes. Hard codes will cause the SERVICE ENGINE SOON light to illuminate continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION in this article. For diagnosing hard codes, proceed to appropriate trouble code chart in this article. For diagnosing intermittent codes, proceed to INTERMITTENTS in H - TESTING W/O CODES article. Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.
3. If no trouble codes were displayed and a driveability problem exists, refer to SYMPTOMS in H - TESTING W/O CODES article. The comments there will send you to the proper system or component to check in SYSTEM/COMP TESTS article.
4. After any repairs are made, clear any trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check in BASIC TESTING article.