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.
"Scan" Diagnostic Circuit Check
The "Scan Diagnostic Circuit Check determines if: 1) the "SERVICE ENGINE SOON" light works, 2) the ECM is operating and can recognize the fault, and 3) if any codes are stored. This is the starting point for any diagnosis. If no codes are indicated, see SYSTEM PERFORMANCE CHECK. If any additional checks are called out from the System Performance Check, see the CCC TESTS W/O CODES article in this section.
- Check operation of the "SERVICE ENGINE SOON" LIGHT. With key on, scanner not connected and engine not running, light should be on steady.
- Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go on and off to indicate a code. If the light goes from bright to dim, see CHART A6. This is not considered a code.
- If the Scanner is not operating, try it on another vehicle. If it works on other vehicle, cigar lighter should be checked for battery voltage and a good ground. If the scanner displays "NO DATA" or "NO ALCL" with the ignition on, see CHART A6.
- No codes at this point indicate the problem is intermittent and the SYSTEM PERFORMANCE CHECK should be performed. If a code or codes is displayed, see appropriate DIAGNOSTIC CHART.
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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.