Malfunction Indicator Lamp: Description and Operation

The check engine lamp should always illuminate with the ignition switch is in the Run position and the engine is Off. When the engine is started, the check engine lamp should turn Off immediately indicating no fault codes have been stored in the ECU. If the lamp turns Off 3 seconds after the engine is started at least one code, known as a secondary fault is stored in the ECU. This type fault will not affect exhaust emissions. If the check engine lamp stays On after the engine is started, at least one code known as a static fault has been stored in the ECU. This type fault could affect any part of the electronic engine control system. Static faults will also cause the ECU to use pre-set values to control engine operation. Only when the fault has been eliminated will the ECU return control to the sensors for operating parameters.