Diagnostic Procedure

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

Ensure all engine systems not related to the computer are operating properly. DO NOT proceed with testing unless all other problems have been repaired. Perform diagnostic circuit check before using trouble code charts. See BASIC TESTING article.
If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes cause SERVICE ENGINE SOON light to illuminate continuously with engine running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION . For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, proceed to INTERMITTENTS in the appropriate TESTS W/O CODES article in the section. Exceptions are Code 13, 15, 24, 44 and 45 charts, which can help diagnose intermittent codes.
If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES article. From there you will be sent to the appropriate SYSTEM/COMPONENT TESTS article.
After repairs are made, clear trouble codes and perform FIELD SERVICE MODE check in BASIC TESTING article.