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.
ECM Check
This check is made to determine why the CEL remains on after engine is started. Normally, the ECM will not recognize a fault for at least 10 seconds after engine start. If CEL remains on, the ECM has lost power, ground, or signal that turns CEL off has been lost. Because the CEL is not part of the ECM, it can recognize ECM faults.
- This step checks for proper CEL signal at ALDL connector. Normally it should be about equal to battery voltage until engine is started.
- This step checks for 12 volts at ECM ignition power terminals. Battery voltage should be measured at both terminals.
- This step ensures good ECM ground. The CEL should normally be on. If ECM power and ground terminals are okay, check for good ECM-to-connector terminal contact.
- When engine is started, the ECM turns the CEL off. Voltage at the ALDL should drop to under 6 volts.
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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.