Hard Failures
Hard failures cause Malfunction Indicator Light (MIL) to illuminate and remain on until problem is repaired. If light comes on and remains on (light may flash) during vehicle operation, cause of malfunction must be determined using diagnostic trouble code charts. If a sensor fails, Electronic Control Module (ECM) will use a substitute value in its calculations to continue engine operation. In this condition, commonly known as limp-in mode, vehicle runs but driveability will not be optimum.
NOTE:
The MIL may also be referred to as the CHECK ENGINE light. The MIL may not illuminate when certain failures such as faulty starter signal, A/C switch signal or if certain trouble codes are set.