Diagnostic Procedure

NOTE: Trouble codes will be recorded at various operating times. Some codes require operation of sensor or switch for 5 seconds; other components require operation for 5 minutes or longer at normal operating temperature, vehicle speed and load. Therefore, some codes may not set in a service bay operational mode and may require road testing vehicle in order to duplicate condition under which code will set.

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 TROUBLE CODES to determine cause of malfunction. See DIAGNOSTIC TESTS for test procedures corresponding to TROUBLE 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 CODES .