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Malfunction Indicator Lamp (MIL) refers to a warning light on the instrument cluster that indicates a malfunction. This warning light indicates almost anything from a loose gas cap to a serious knock in the engine.

  • When certain troubles develop in the electronic control system. the electronic control module stores in its memory a number of trouble code for each fault. This turns on a malfunction-indicator lamp in the instrument panel. A check engine service engine soon or similar light alerts the driver that service is needed.
  • If a single sensor fails, the electronic control module may substitute a value for the failed sensor. This allows the engine to appear to run normally. A fault that turns on the malfunction indicator light may also put the engine into its limp-in mode. This means that the limited operation strategy chip in the electronic control module has taken over. It provides basic instruction to the microprocessor if part of the electronic system fails.
  • The engine runs, but with fixed igniton timing and air-fuel ratio. The injector pulse width does not change. This allows the vehicle to be driven but with greatly reduced performance. The vehicle can limp in for service instead of being towed.