Diagnostic Procedure

NOTE: DTCs will be recorded at various operating times. Some DTCs 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 DTC may not set in a service bay and may require road testing vehicle to duplicate the condition under which the DTC sets.

Diagnosis of computerized engine control system should be performed in the 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 on, see RETRIEVING DTC(S) . For diagnosing hard codes, see DIAGNOSTIC TROUBLE CODE (DTC) DEFINITION under DIAGNOSTIC TESTS to determine cause of malfunction.
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 appropriate wiring diagram in WIRING DIAGRAMS section 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 DTC still exists, substitute with a known-good ECM. After repairs have been completed, clear DTCs. See CLEARING DTC(S) .