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) operates.
- If PCM is operating and can recognize a fault.
- If any DTCs are stored.
After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS and BASIC FUEL 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 diagnostic 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. If Malfunction Indicator Light (MIL) illuminates, go to next step. If MIL does not illuminate, go to MIL INOPERATIVE.
- 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 PCM data, go to next step. If scan tool does not display PCM 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 MIL ON STEADY.
- Using scan tool, check if any DTCs are present. If DTCs are present, go to next step. If DTCs are not present, go to step 6).
- Using scan tool, store FREEZE FRAME and FAILURE RECORDS data. Diagnose any stored DTC. See the TESTS W/CODES article.
- 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 .
- Turn ignition on with engine off. Check Engine Coolant Temperature (ECT) and TP sensors for proper operation. Start and warm engine to normal operating temperature. Check ECT sensor, MAP sensor, O2 sensors, and IAC valve for proper operation. Compare scan tool engine data with actual control system data values. If values are within limits, see the TESTS W/O CODES article. If values are 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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