Operation

OPERATION

The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when the Engine Control Module (ECM) (also known as the Powertrain Control Module/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 instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster over the Controller Area Network (CAN) data bus from the ECM.

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 remains 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 an electronic MIL lamp-ON message from the ECM, the indicator will be illuminated. The indicator will remain illuminated until the detected DTC is repaired and a lamp-OFF message is received from the ECM, or until the ignition switch is turned to the OFF position, whichever occurs first. For more information on the ECM and the DTC set and reset parameters, Emissions Control - Operation.
- Communication Error - If the cluster receives no lamp-ON or lamp-OFF messages 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 ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN or KOMBI). If the ECM sends a lamp-ON message after the bulb test, it indicates that the ECM has detected an engine, fuel, exhaust or emissions system fault and that these systems may require service.

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