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 - Emissions related, turns on the Malfunction Indicator Lamp (MIL) the first time the diagnostic executive reports a fault.
^ Type B - Emissions related, turns on the MIL if fault is active for 2 consecutive driving cycles.
^ Type C Non-emissions related, does not turn on the MIL but will turn on the service light.
^ Type D - Non -emission related does not turn on 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% 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 Diagnostic Trouble Code (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.