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, 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 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.
- Check if Malfunction Indicator Light (MIL) is illuminated with engine running, for driveability concerns, or faulty computer-controlled components. If problem is found, go to next step.
- Ensure battery is fully charged. 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 .
- Attempt to start engine. If engine start and runs, go to next step. If engine does not start, or starts and dies, go to NO START DIAGNOSIS .
- Using scan tool, observe DTC status: MIL REQUEST, FAIL THIS IGN, LAST TST FAIL and HISTORY. If any DTC status is present, refer to affected DTC to diagnose problem. See the TESTS W/CODES article. If DTCs are not present, go to next step.
- Compare scan tool engine data with actual control system data values. If value is within limits, see the TESTS W/O CODES article. If value is not within limits, go to the SYSTEM/COMPONENT TESTS article.
NO RELATED
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Affiliate disclosure: We may earn a commission at no extra cost to you.
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.