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: Notes
When ignition is first turned on, Malfunction Indicator Light (MIL) will illuminate for approximately 2 seconds to verify bulb and circuit operation, and then go off. The MIL (CHECK ENGINE light) is located on instrument panel.
When Powertrain Control Module (PCM) detects a Diagnostic Trouble Code (DTC) that affects vehicle emissions, MIL is illuminated. If a problem is detected by PCM, PCM sends a message over a Bus line to turn on MIL in instrument cluster.
Specific criteria must be met for PCM to store a DTC. Criteria may be a specific engine RPM range, coolant temperature or input voltage to PCM. PCM may not store a DTC even if a malfunction occurred. This may happen if specific criteria was not met for setting a DTC. As an example, if DTC criteria requires PCM to monitor a circuit only when engine is at 750-2000 RPM and a sensor is shorting when engine is operating at more than 2000 RPM resulting in zero voltage input to PCM, PCM will not store a DTC because malfunction occurred outside DTC criteria.
PCM will only illuminate MIL for failures which affect vehicle emissions. If MIL is on continuously, PCM has entered a limp-in mode, identified a failed emission component or system. MIL will remain on until cleared using scan tool. MIL will either remain on continuously or flash when PCM detects an active engine misfire. PCM will turn off MIL if:
- PCM does not detect the malfunction for 3 consecutive trips (except for misfire and fuel system monitors).
- PCM does not detect a malfunction while performing 3 consecutive engine misfire or fuel system tests. PCM performs these tests when engine is operating within plus or minus 375 RPM of and within 10 percent engine load of operating condition when malfunction was first detected.
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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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