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.
On-Board Diagnostics
The Engine Control Module (ECM) monitors several different engine control system circuits. If malfunction occurs, ECM will store a Diagnostic Trouble Code (DTC) when malfunction is detected after engine is restarted on all failures except engine misfire or excessive time to enter closed loop. On engine misfire or excessive time to enter closed loop, ECM will store a DTC when malfunction is first detected. When DTC is stored, ECM will use a substitute value for the failed component in its calculations to continue engine operation. In this condition, commonly know as limp-in mode, the vehicle runs but driveability will be optimum.
The ECM also records and stores engine operating conditions when the malfunction occurred. This information is referred to as freeze frame data. Freeze frame data may be obtained using scan tool, and used for simulating tests for trouble shooting. Freeze frame data is available for:
- Engine Coolant Temperature
- Engine Speed
- Vehicle Speed
- Long Term Fuel Compensation
- Short Term Fuel Compensation
- Fuel Control Condition Such As Open Or Closed Loop
- Calculation Load Value
- DTC Data Recording
DTCs may be retrieved for system diagnosis. System malfunctions are identified as either hard failures or intermittent failures.
Use the Manual With the Right Hardware
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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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