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.
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 TESTS W/O CODES article and SCAN TESTER usage in 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 A-1, MALFUNCTION INDICATOR LIGHT (MIL) .
- Turn ignition off. Install Tech 1 scan tool, 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 ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK .
- If DTC(s) are present, diagnose DTC(s) using appropriate self-diagnostic test. See TESTS W/CODES article. If DTC(s) are not present, go to next step.
- Turn ignition off for 20 seconds. Turn ignition on and wait for glow plug cycle. Crank engine for about 10 seconds. If engine start and runs, go to next step. If engine does not start or starts and dies, go to A-3, NO-START DIAGNOSIS .
- Compare scan tool engine data with actual control system data values. If parameter is within limits, go TESTS W/O CODES article. If parameter is not within limits, go to next step.
- Go to SYSTEM/COMPONENT TESTS article. Replace components that are out of range.
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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.