Diagnostic Procedure

Diagnosis of computerized engine control system should be performed in 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 the BASIC TESTING article.
2. If DTC(s) were displayed, determine whether codes are hard or intermittent trouble codes. 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. For diagnosing intermittent codes, proceed to INTERMITTENTS in the TESTS W/O CODES article.
3. If no DTCs are present and a driveability problem exists, refer to SYMPTOMS in the TESTS W/O CODES article. Doing so will help identify proper system or component to check in the SYSTEM/COMPONENT TESTS article.
4. After necessary repairs are made, clear DTCs, verify vehicle will enter "closed loop" operation and ensure DTC does not reset.