Diagnosis of computerized engine control system should be performed in following order:
- Ensure all engine systems other than computerized systems (i.e. compression, ignition, etc.) are operating properly. DO NOT proceed with testing unless all other problems have been repaired. If MIL light is illuminated, see RETRIEVING CODES (NON-SCAN) or RETRIEVING CODES (SCAN TESTER). For diagnosing hard codes, see FAULT CODES to determine cause of malfunction. See DIAGNOSTIC TESTS for test procedures corresponding to fault codes.
- For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article. Inspect condition of system harness by conducting recommended voltage or continuity checks. See WIRING DIAGRAMS article for additional information on specific circuit configuration. Inspect condition of individual component. Repair or replace component as needed.
- Recheck system operation after repair is made. If trouble code still is present, substitute ECM with known good unit. Verify that system is repaired. After repairs have been completed, clear trouble codes. See CLEARING TROUBLE CODES.