Hard Failures
Hard failures cause Malfunction Indicator Light (MIL) on instrument panel to illuminate and remain on until problem is repaired. If light comes on and remains on (light may flash) during vehicle operation, corresponding trouble code will be retained in Electronic Control Module (ECM) memory on some trouble code applications. Not all trouble codes are retained in ECM memory. The cause of malfunction must be determined using 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, the 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 failure such as faulty starter signal or A/C switch signal exists, or if certain trouble codes are set.