Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
On-Board Diagnostic (OBD) System Check: Notes
The OBD System Check determines:
- If Malfunction Indicator Light (MIL) works.
- If PCM/VCM is operating and can recognize a fault.
- If any Diagnostic Trouble Codes (DTCs) are stored.
After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS, this is the starting point for utilizing the self-diagnostic system for determining computer-related problems. After performing necessary tests as described in the OBD system circuit check, if no codes are indicated and driveability problems still exist, see the TESTS W/O CODES article and SCAN TOOL usage in the TESTS W/CODES article.
- 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 MALFUNCTION INDICATOR LIGHT (MIL) .
- Turn ignition off. Install Tech 1 scan tool and follow scan tool manufacturer's instructions to proceed with test. Turn ignition on. If scan tool displays VCM data, go to next step. If scan tool does not display VCM data, go to DLC DIAGNOSIS OR NO SCAN TOOL DATA .
- Using scan tool, command MIL to turn off. If MIL turns off, go to next step. If MIL does not turn off, go to MALFUNCTION INDICATOR LIGHT (MIL) .
- Using scan tool, observe DTC status for MIL REQUEST, FAIL THIS IGN, LAST TST FAIL and HISTORY. If any DTC is stored, save freeze frame and fail record information using scan tool CAPTURE INFO feature. If any of these DTC status are present, refer to affected DTC to diagnose problem. If DTC(s) are not present, go to next step.
- Crank engine for 10 seconds. If engine starts, go to next step. If engine does not start, go to NO-START DIAGNOSIS .
- Compare scan tool engine data with actual control system data values. If value is within limits, go to the TESTS W/O CODES article. If value is not within limits, go to the SYSTEM/COMPONENT TESTS article.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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