Diagnostic Procedure

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

1. 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 - 7.4L article.
2. If trouble codes were displayed (other than Code 12), determine whether codes are hard or intermittent. Hard codes may cause Malfunction Indicator Light (MIL) to illuminate continuously with engine running. For diagnosing hard codes, proceed to appropriate trouble code chart. For diagnosing intermittent codes, use DIAGNOSTIC AIDS in appropriate code chart or proceed to INTERMITTENTS in TESTS W/O CODES - GASOLINE article.
3. If trouble codes were not displayed and a driveability problem exists, refer to SYMPTOMS in TESTS W/O CODES - GASOLINE article in the ENGINE PERFORMANCE section. From there you will be sent to SYSTEM/COMPONENT TESTS - 7.4L article.
4. After repairs are made, clear trouble codes and perform FIELD SERVICE MODE CHECK in BASIC TESTING - 7.4L article.