Malfunction Indicator Lamp

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 a transistor on the IC circuit board based upon programming and electronic messages received by the IC from the PCM over the CAN data bus. The IC will illuminate the MIL indicator for about fifteen seconds as a bulb test each time the ignition switch status transitions to ON.

The MIL LED is completely controlled by the IC logic circuit, and that logic will only allow this indicator to operate when the IC receives a battery current input on the fused ignition output (run-start) circuit. Therefore, the LED will always be OFF when the ignition switch status is anything except ON or START. The LED only illuminates when it is provided a path to ground by the IC transistor. The IC will turn ON the MIL for the following reasons:

• MIL Lamp-On Message  - Each time the IC 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. Refer to Operation .
• Communication Error  - If the IC receives no lamp-ON or lamp-OFF messages from the PCM for 10 consecutive message cycles, the MIL is illuminated by the IC to indicate a loss of bus communication. The indicator remains controlled and illuminated by the IC 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 IC. If the IC 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.

For proper diagnosis of the fuel and emissions systems, the PCM, the IC, the CAN data bus or the electronic communication related to MIL operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.