Malfunction Indicator Lamp: Description and Operation

MALFUNCTION INDICATOR LAMP (MIL) OPERATION
The malfunction indicator lamp (MIL) is located in the instrument panel cluster. The MIL will display as either "SERVICE ENGINE SOON" or one of the following symbols when commanded ON:









The Malfunction Indicator Lamp (MIL) is on the instrument panel. The MIL has the following functions:
^ The MIL informs the driver that a fault has occurred that affects the vehicle emission levels. The owner should take the vehicle in for service as soon as possible.
^ As a bulb and system test, the MIL comes on with the key-on and the engine not running. When the engine is started, the MIL turns off if no DTCs are set.

When the MIL remains on while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, perform a Powertrain On-Board Diagnostic (OBD) System Check. These tests expose faults which the technician may not detect if other diagnostics are performed first.

MIL REQUESTED
This message displayed indicates that the DTC is currently causing the MIL to turn on. Remember that only type A and B DTCs can request the MIL. The MIL request cannot determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive requires up to 3 trips during which the diagnostic test passes to turn off the MIL.

MIL REQUESTS AND HISTORY CODES
The diagnostic executive must acknowledge when all the emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into 4 types:
^ Type A-Is emissions related, and turns on the MIL the first time the diagnostic executive reports a fault.
^ Type B-Is emissions related, and turns on the MIL if the fault is active for 2 consecutive driving cycles.
^ Type C-Is non-emissions related, and does not turn on the MIL, but will turn on the service light.
^ Type X- Is non -emission related and does not turn on the MIL or the service light.

When a type A diagnostic test reports a failure, the diagnostic executive immediately requests to have the MIL turn on for that diagnostic test. When a type B diagnostic test reports a failure during 2 consecutive trips, the diagnostic executive turns on the MIL for that diagnostic test. The diagnostic executive has the option of turning the MIL off when the diagnostic test which caused the MIL to illuminate passes for 3 consecutive trips. In the case of misfire or fuel trim malfunctions, there are additional requirements as follows:
^ The load conditions must be within 10 percent of the vehicle load present when the diagnostic executive reported the failure.
^ The engine speed conditions must be within 375 RPM of the engine speed present when the diagnostic executive reported the failure.
^ The engine coolant temperature must have been in the same range present when the diagnostic executive reported the failure.

When the diagnostic executive requests the service light to be turned on, or a type C diagnostic fault is reported, a history DTC is also recorded for the diagnostic test. The provision for clearing a history DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail, a battery disconnect, or a scan tool clear info command.