MIL Requests and History Codes

The Diagnostic Executive must be able to acknowledge when all emissions related diagnostic tests have reported a pass or fail condition since the last ignition cycle. Each diagnostic test is separated into four types:

^ A emissions related, turns "ON" the MIL the first time the diagnostic executive reports a fault.
^ B emissions related, turns "ON" the MIL if active for 2 consecutive driving cycles.
^ C non-emissions related, does not turn "ON" the MIL but will turn on the service light.
^ 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 two 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 be illuminated passes for three 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 MIL 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 erasing (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.