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 System Check
On-Board Diagnostic (OBD) System Check is an organized approach in identifying a problem created by an electronic engine system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to next logical step in diagnosing a complaint. Understanding and using it correctly will reduce diagnostic time and prevent unnecessary replacement of good parts.
- Check for one or more of the following conditions: The Malfunction Indicator Light (MIL) illuminates with engine running. A customer concern of engine performance or driveability. A suspected fault in a PCM controlled component or system. If any of these conditions are present, go to next step. If none of these conditions are present, system is okay.
- Turn ignition on, with engine off. Observe MIL. If MIL is illuminated, go to next step. If MIL is not illuminated, diagnose MIL. See MALFUNCTION INDICATOR LIGHT INOPERATIVE .
- Turn ignition off. Install scan tool. Turn ignition on, with engine off. If scan tool displays any PCM data, go to next step. If scan tool does not display any PCM data, see DATA LINK CONNECTOR DIAGNOSIS OR NO SCAN TOOL DATA .
- Start engine. If engine starts and runs, go to next step. If engine cranks but does not start, see NO START DIAGNOSIS in appropriate BASIC DIAGNOSTIC PROCEDURES.
- Recheck DTCs. If any DTCs are set, save DTC and FREEZE FRAME/FAILURE RECORD information. Diagnose affected DTC(s). If no DTCs are set, go to next step.
- Check if MIL is illuminated with no DTCs set. If MIL is illuminated, go to MALFUNCTION INDICATOR LIGHT ON STEADY . If MIL is not illuminated, go to next step.
- Compare scan tool data display with typical scan tool data values. If values are normal or within limits, see appropriate TROUBLE SHOOTING - NO CODES article. If values are not within limits, go to appropriate SYSTEM & COMPONENT TESTING article.
Use the Manual With the Right Hardware
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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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