Powertrain On-Board Diagnostic (OBD) System Check

The OBD System Check determines:

• If Malfunction Indicator Light (MIL) operates.
• If PCM is operating and can recognize a fault.
• If any DTCs are stored.

After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS in BASIC ENGINE DIAGNOSTIC PROCEDURES article, this is the starting point for utilizing the self-diagnostic system for determining computer-related problems. After performing necessary tests as described in the diagnostic circuit check, if no codes are indicated and driveability problems still exist, see ENGINE CONTROLS - TESTS W/O CODES article and SCAN TOOL USAGE.

1. Check if Malfunction Indicator Light (MIL) is illuminated with engine running, for driveability concerns, or faulty computer-controlled components. If problem is found, go to next step.
2. Ensure battery is fully charged. Turn ignition off. Install scan tool and follow scan tool manufacturer's instructions to proceed with test. Turn ignition on. If scan tool displays PCM data, go to next step. If scan tool does not display PCM data, go to DLC DIAGNOSIS OR NO SCAN TOOL DATA.
3. Attempt to start engine. If engine starts and runs, go to next step. If engine does not start, or starts and dies, go to NO START DIAGNOSIS in BASIC ENGINE DIAGNOSTIC PROCEDURES article.
4. Using scan tool, observe DTC status: MIL REQUEST, FAIL THIS IGN, LAST TST FAIL and HISTORY. If any DTC is present, refer to affected DTC to diagnose problem. If DTCs are not present, go to next step.
5. Compare scan tool engine data with actual control system data values. If data value is within limits, see ENGINE CONTROLS - TESTS W/O CODES article. If data value not within limits, go to ENGINE CONTROL SYSTEM & COMPONENT TESTS article.