Malfunction Indicator

WARNING: This page is about a different car, the 2012 RAM Pickup and 2012 RAM Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

The MIL gives an indication to the vehicle operator when the PCM or the PCM has recorded a Diagnostic Trouble Code (DTC) for an On-Board Diagnostics II (OBDII) emissions-related circuit or component malfunction. The MIL is controlled by a transistor on the CCN circuit board based upon cluster programming and electronic messages received over the Controller Area Network (CAN) data bus. These electronic messages are received from the PCM or PCM, depending on engine application.

The MIL Light Emitting Diode (LED) is completely controlled by the CCN logic circuit, and that logic will only allow this indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED will always be OFF when the ignition switch is in any position except ON or START. The LED only illuminates when it is provided a path to ground by a transistor on the CCN circuit board. Each time the ignition switch is turned to the ON position the indicator is illuminated for about two seconds as a bulb test. The entire bulb test is a function of the PCM. The MIL will be turned ON for the following reasons:

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

The PCM or ECM continually monitors the fuel and emissions system circuits and sensors to decide whether the system is in good operating condition. The PCM or ECM then sends the proper lamp-ON or lamp-OFF message to the CCN. For further diagnosis of the MIL or the instrument cluster circuitry that controls the LED, refer to DIAGNOSIS AND TESTING . If the CCN turns ON the MIL after the bulb test, it may indicate that a malfunction has occurred and that the fuel and emissions systems may require service.