Malfunction Indicator Lamp: Description and Operation
The Malfunction Indicator Lamp (MIL) gives an indication to the vehicle operator when 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 from the PCM 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 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 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 indicator is illuminated for about 15 seconds as a bulb test. The entire
bulb test is a function of the PCM.
MIL Lamp-On Message - Each time the cluster 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: Powertrain Management/Computers and
Control Systems/Testing and Inspection/Monitors, Trips, Drive Cycles and Readiness Codes.
Communication Error - If the cluster receives no lamp-ON or lamp-OFF messages from the PCM for 10 consecutive message cycles, 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.
Actuator Test - Each time the cluster is put through the actuator test, the MIL indicator will be turned ON during the bulb check portion of the
test to confirm the functionality of the LED and the cluster control circuitry.
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 ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment
Node/CCN). If the EMIC 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 PCM, 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.