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 A2 - No Code 12 "Check Engine Soon" Light On
"CHECK ENGINE" light should be "ON" steady when ignition is "ON" and engine is "OFF". Battery voltage is supplied to the bulb, bulb is grounded by ECM through circuit 419.
With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble codes stored in memory. A steady light is possibly a short to ground in circuit 419, or an open in diagnostic circuit 451.
- If the light goes "OFF" when the ECM connector is disconnected, circuit 419 is not shorted to ground. Check connector terminals for proper contact.
- This step checks for open diagnostic circuit 451.
- At this point, light wiring is okay. Problem is a faulty ECM or PROM. ECM is okay if Code 51 is stored when PROM is removed.
- Relays and switches are operated by the ECM, using internal switches called "Drivers". Each driver is part of a group of 4 called "Quad-Drivers". Failure of any driver can damage any other driver in the set. Use an ohmmeter to check solenoids listed in 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.