Diagnostic Procedure

NOTE: DTCs will be recorded at various operating times and conditions. 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 DTCs may not set in a service bay, and may require road testing to duplicate the conditions under which the code will set.

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 is on, see RETRIEVING CODES. For diagnosing hard codes, see DTC IDENTIFICATION table under DIAGNOSTIC TROUBLE CODE (DTC) DEFINITIONS to determine the probable cause of malfunction.
For diagnosing intermittent codes, proceed to INTERMITTENTS in TESTS W/O CODES article. Inspect condition of system harness by performing 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 completed. If DTC still exists, substitute known-good ECM, then retest system. Verify system is repaired. After repairs have been completed, clear DTC(s). See CLEARING CODES .