Malfunction Indicator Lamp: Description and Operation

MALFUNCTION INDICATOR LAMP (MIL)

DESCRIPTION
A Malfunction Indicator Lamp (MIL) is standard equipment on all instrument clusters. The MIL is located near the lower edge of the instrument cluster, to the right of the multi-function indicator display. The MIL consists of the International Control and Display Symbol icon for "Engine" imprinted within a rectangular cutout in the opaque layer of the instrument cluster overlay. The dark outer layer of the overlay prevents the indicator from being clearly visible when it is not illuminated. An amber Light Emitting Diode (LED) behind the cutout in the opaque layer of the overlay causes the icon to appear silhouetted against an amber field through the translucent outer layer of the overlay when the indicator is illuminated from behind by the LED, which is soldered onto the instrument cluster electronic circuit board. The MIL is serviced as a unit with the instrument cluster.

OPERATION
The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when the Engine Control Module (ECM) 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 instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the ECM over the Controller Area Network (CAN) data bus. The MIL Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit, and that logic will only allow this indicator to operate when the instrument cluster detects that the ignition switch is in the On position. Therefore, the LED will always be off when the ignition switch is in any position except On. The LED only illuminates when it is provided a path to ground by the instrument cluster transistor. The instrument cluster will turn on the MIL for the following reasons:

- Bulb Test - Each time the ignition switch is turned to the On position the MIL is illuminated as a bulb test. The indicator will remain illuminated until the engine is started, or until the ignition switch is turned to the Off position, whichever occurs first.
- MIL Lamp-On Message - Each time the cluster receives a MIL lamp-on message from the ECM, the indicator will be illuminated. The indicator will remain illuminated until the detected fault is repaired, or until the ignition switch is turned to the Off position, whichever occurs first.
- Communication Error - If the cluster receives no lamp-on or lamp-off message from the ECM, the MIL is illuminated by the instrument cluster. The indicator remains controlled and illuminated by the cluster until a valid lamp-on or lamp-off message is received from the ECM.

The ECM continually monitors the fuel and emissions system circuits and sensors to decide whether the system is in good operating condition. The ECM then sends the proper lamp-on or lamp-off messages to the instrument cluster. If the instrument cluster 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. For proper diagnosis of the fuel and emissions systems, the ECM, the CAN data bus, or the electronic message inputs to the instrument cluster that control the MIL, a diagnostic scan tool is required. Refer to the appropriate diagnostic information.