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 TESTING 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 TESTS W/O CODES article and SCAN TOOL USAGE.

1. Turn ignition on with engine off. Observe Malfunction Indicator Light (MIL). If MIL illuminates, go to next step. If MIL does not illuminate, go to MIL INOPERATIVE.
2. 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 TESTING article.
4. Using scan tool, observe DTC status on LAST TST FAIL function. If any DTC is stored, save freeze frame and fail record information using scan tool CAPTURE INFO feature. If a DTC is indicated as last test failed, diagnose affected DTC. If DTC is not indicated, go to next step.
5. Using scan tool, display DTC fail records. If a DTC fail record is stored, save freeze frame and fail record information using scan tool CAPTURE INFO feature. If a fail record is not stored, go to next step. If a fail record is stored, diagnose affected DTC.
6. Compare scan tool engine data with actual control system data values. If value is within limits, system is okay. If value is not within limits, go to SYSTEM/COMPONENT TESTS article.