Diagnostic Procedure

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

1. Ensure all engine systems not related to computer system are operating properly. DO NOT proceed with testing unless all other problems have been repaired. On-Board Diagnostic (OBD) system check must be performed before using specific DTC testing procedure. See POWERTRAIN DIAGNOSTIC SYSTEM CHECK  under DIAGNOSTIC SYSTEM CHECKS.
2. If DTCs were displayed, determine whether codes are hard or intermittent. Hard codes will cause MIL to illuminate continuously while engine is running. See HARD OR INTERMITTENT TROUBLE CODE DETERMINATION . For diagnosing hard codes, proceed to appropriate DTC test. See DIAGNOSTIC TESTS . For diagnosing intermittent codes, see appropriate TROUBLE SHOOTING - NO CODES article.
3. If no DTCs are present and a driveability problem exists, refer to SYMPTOMS in appropriate TROUBLE SHOOTING - NO CODES article. Doing so will help identify proper system or component to check in appropriate SYSTEM & COMPONENT TESTING article.
4. After necessary repairs are made, clear DTCs, verify vehicle will enter "closed-loop" operation and ensure DTC does not reset.