MIL Requests and History Codes
MIL REQUESTS AND HISTORY CODESThe 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 D - is non -emission related and does not turn ON the MIL or the service light.
DIAGNOSTIC EXECUTIVE / TYPE A, B, & C DIAGNOSTIC TESTS
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 the 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 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.
SPECIAL CASES OF TYPE A DIAGNOSTIC TESTS
Unique to the misfire diagnostic, the Diagnostic Executive has the capability of alerting the driver of potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter, the Diagnostic Executive will command the MIL to flash at a rate of once per second during the times that the catalyst damaging misfire condition is present.
SPECIAL CASES OF TYPE B DIAGNOSTIC TESTS
Misfire and fuel trim malfunctions are special cases of type B diagnostics. Each time a fuel trim malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.
When the ignition is turned off, the last reported set of conditions remain stored. During subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a fuel trim malfunction occurs during 2 consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic. The Diagnostic Executive does not use the stored conditions. However, if a fuel trim malfunction occurs on 2 non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:
^ When the engine load conditions are within 10% of the previous test that failed.
^ The engine speed is within 375 RPM of the previous test that failed.
^ The engine coolant temperature is in the same range as the previous test that failed.