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 (code) charts. If a sensor fails, PCM 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. When a malfunction is detected in PCM, fuel injection is controlled by an independent back-up system, permitting minimal driving.

To prevent a false Diagnostic Trouble Code (DTC) from being stored in PCM memory, some malfunctions must be detected during 2 consecutive drive cycles. This applies to heated oxygen sensors (HO2S), fuel metering-related components, idle control system, Engine Coolant Temperature (ECT) sensor, Exhaust Gas Recirculation (EGR) system, Three-Way Catalytic (TWC) converter system, evaporative (EVAP) control system and other self-diagnostic functions. First occurrence of a detected malfunction is stored in PCM memory as a temporary code. If same malfunction is detected again after ignition switch is turned off and then back on, a hard failure DTC is stored and Malfunction Indicator Light (MIL) is turned on. Malfunction can be determined by retrieving DTCs.