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.
Chart A-2, Check Engine Light On At All Times, Will Not Flash Code 12: Notes
CHECK ENGINE light will flash diagnostic trouble codes when the diagnostic request terminal in the duty check coupler is grounded. On vehicles with Federal emissions, the CHECK ENGINE light is also activated by a mileage switch at 50,000, 80,000 and 100,000 miles, alerting operator that vehicle is in need of regular maintenance service. Once service is performed, CHECK ENGINE light should be reset. Reset switch is located behind instrument panel, attached to left speaker.
- Checks for a short to ground in Purple/Yellow wire.
- Checks for a faulty ECM or an open in Blue/Yellow and Black wires to duty check coupler.
- Checks for short to ground in instrument panel printed circuit.
- Checks to see if CHECK ENGINE light cancel switch or mileage switch is faulty.
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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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