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OBD II Basics
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MIL Fault Codes
MIL fault codes can result from problems with the electrical wiring, certain sensors and actuators, excessive fuel corrections, engine misfire, and failure of secondary emissions components such as those used in the EGR system, Secondary Air Injection system, Catalyst, Oxygen Sensor, Oxygen Sensor Heater, and Evaporative system.

When a fault is detected a Diagnostic Trouble Code (DTC) is activated and the MIL is turned on.

Once the OBD II system in the vehicle turns on the MIL, it leaves it on to remind the driver to repair the fault.

Once the OBD II system in the vehicle turns on the MIL, it stays on to  remind the driver to obtain further diagnosis by a skilled automotive repair technician to determine the most appropriate repairs.

The MIL can be turned off in one of three ways:
  • The vehicle computer can turn off the MIL if the problem does not reoccur under the conditions where the problem was previously detected. 
  • The MIL can be turned off by erasing DTCs with a scan tool.
  • The MIL can be turned off if computer battery power is interrupted, commonly when the vehicle battery is disconnected or goes dead.
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