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.
Diagnostic Trouble Codes
The Engine Control Module (ECM) or Powertrain Control Module (PCM) monitors several different engine control circuits. If a malfunction occurs, ECM will record a Diagnostic Trouble Code (DTC). The next time engine is started and the same malfunction is detected (under similar conditions), ECM/PCM will turn on MIL, signaling DTC is in ECM/PCM memory.
After 3 consecutive drive cycles without a malfunction, ECM/PCM will turn off MIL, however DTC will be stored in ECM/PCM memory. If ECM/PCM does not detect that fault for 40 drive cycles, DTC will be cleared from ECM/PCM memory . ECM/PCM will clear a DTC for a misfire or fuel trim malfunction when fault is not detected for 80 drive cycles.
Hard faults cause MIL to illuminate and remain on until the malfunction is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined. If a sensor fails, ECM/PCM will use a substitute value in its calculations to continue engine operation. See FAIL-SAFE MODES under SELF-DIAGNOSTIC SYSTEM. In this condition, vehicle is functional, but driveability will be affected.
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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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