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.
"Non-Scan" Diagnostic Circuit Check
The "Non-Scan" Diagnostic Circuit Check determines if: 1) the "SERVICE ENGINE SOON" light works, 2) the ECM is operating and can recognize a fault, and 3) if any codes are stored. It also checks to see if stored codes indicate an intermittent problem. This is the starting point for any diagnosis. If no codes are indicated, see SYSTEM PERFORMANCE CHECK. If no additional checks are called out from the System Performance Check, see TROUBLE SHOOTING charts in the CCC TESTS W/O CODES article in this section.
- Check operation of the "SERVICE ENGINE SOON" light. Key on 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.
- This step will determine if any codes, other than Code 12, are still present or were intermittent and are no longer stored. Clear memory. Run vehicle for 2 minutes. See if trouble code(s) reset.
- If light is on, fault is still present. See applicable trouble code chart.
- If light is off, fault is either intermittent, or it is a code that cannot be set with vehicle stationary. For codes that cannot be set during the Diagnostic Circuit Check, the applicable trouble code chart will determine if those codes are intermittent.
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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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