Malfunction Indicator

The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when the PCM has recorded a DTC for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. The MIL is controlled by the IPC circuit board based upon IPC programming and electronic messages received by the IPC from the PCM over the CAN data bus.

The MIL is completely controlled by the IPC logic circuit, and that logic will only allow this indicator to operate when the IPC receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the indicator will always be OFF when the ignition switch is in any position except ON or START. The indicator only illuminates when it is energized by the IPC logic circuit. Each time the ignition switch is turned to the ON position the indicator is illuminated for about 15 seconds as a bulb test. The entire bulb test is a function of the PCM. The IPC will turn ON the MIL for the following reasons:

• MIL Lamp-On Message  - Each time the IPC receives an electronic MIL lamp-ON  message from the PCM, the indicator will be illuminated. The indicator can be flashed ON and OFF, or illuminated solid, as dictated by the PCM message. For some DTCs, if a problem does not recur, the PCM will send a lamp-OFF message automatically. Other DTCs may require that a fault be repaired and the PCM be reset before a lamp-OFF message will be sent. For more information on the PCM, and the DTC set and reset parameters, see the appropriate emissions service information.
• Communication Error  - If the IPC receives no lamp-ON or lamp-OFF messages from the PCM for 10 consecutive message cycles, the MIL is illuminated by the IPC to indicate a loss of bus communication. The indicator remains controlled and illuminated by the IPC until a valid message is received from the PCM.

The PCM continually monitors the fuel and emissions system circuits and sensors to decide whether the system is in good operating condition. The PCM then sends the proper lamp-ON or lamp-OFF message to the IPC. If the IPC turns ON the MIL after the bulb test, it may indicate that a malfunction has occurred and that the fuel and emissions systems require service.