MIL Requests and History DTCS

MIL REQUESTS AND HISTORY DTC(s)
(This section explains what happens when type "A B" and "D" Diagnostic Trouble Code (DTC)s report failures and passes, and how the Malfunction Indicator Lamp (MIL) responds. Includes criteria for turning light "ON" and "OFF.")

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 categories:

A. Emissions related, turn "ON" MIL,
B. Emissions related, turn "ON" MIL if active for 2 consecutive driving cycles,
D. Non-emissions related, do not turn "ON" MIL.

When a type "A" diagnostic test reports a failure, the diagnostic executive initiates a request to have the MIL turn "ON" for that diagnostic test. When a type B" diagnostic test reports a second failure, the diagnostic executive initiates a request for that diagnostic test.

The diagnostic executive has the option of turning the MIL "OFF," when three consecutive trips are recorded where the diagnostic system passes the test. In the case of misfire or fuel system malfunctions there is a additional requirement that the load conditions must be within 10%, the speed conditions must be within 375 RPM, and the coolant temperatures must be in the same calibratable high or low range at the time the diagnostic test last reported a failure.

For a type "D" diagnostic test the diagnostic executive will not request that the MIL be turned "ON."

When the diagnostic executive requests the MIL to be turned "ON" a history DTC is also recorded to the diagnostic test. The provision for erasing (clearing) a history Diagnostic Trouble Code (DTC) for type "A" or "B" diagnostic tests involves 40 subsequent warm-up cycles during which no type "A" or "B" diagnostic tests have reported a fail.

SPECIAL CASES OF TYPE "B" DIAGNOSTIC TESTS
Misfire or fuel system malfunctions are special cases or type "B" diagnostics. Each time a misfire or fuel system malfunction is detected, engine load, speed, and coolant conditions are recorded. When the ignition is turned "OFF," the last reported set of conditions before the key-off are stored. On the next subsequent ignition cycle the stored conditions are used as a comparative reference for similar conditions.

If a misfire or fuel system malfunction occurs within two consecutive trips, the Diagnostic Executive treats the failure as a normal type "B" diagnostic, and does not use the stored conditions. However, if a misfire or fuel system malfunction on two non-consecutive trips, the stored conditions are compared with the current conditions. When the load conditions are within 10%, the speed conditions are within 375 RPM, and the coolant temperatures is in the same calibratable high or low range at the time the diagnostic test last reported a failure, the MIL is enabled. At the end of the second trip, if similar conditions were never encountered, the saved conditions are updated to those most recently recorded.

Unique to the "misfire" diagnostic, is a special set of circumstances which may illuminate the MIL during the first "testfailed" reported to the Diagnostic Management System: If the misfire conditions are such that a potential exists to damage the Catalytic Converter as a result of high misfire levels, the Diagnostic Management System will command the MIL to "flash" at a 1 hertz rate during that ignition cycle.

SPECIAL CASES OF TYPE "B" DIAGNOSTIC TESTS
Misfire or fuel system malfunctions are special cases or type "B" diagnostics. Each time a misfire or fuel system malfunction is detected, engine load, speed, and coolant conditions are recorded. When the ignition is turned "OFF," the last reported set of conditions before the key-off are stored. On the next subsequent ignition cycle the stored conditions are used as a comparative reference for similar conditions.

If a misfire or fuel system malfunction occurs within two consecutive trips, the Diagnostic Executive treats the failure as a normal type "B" diagnostic, and does not use the stored conditions. However, if a misfire or fuel system malfunction on two non-consecutive trips, the stored conditions are compared with the current conditions. When the load conditions are within 10%, the speed conditions are within 375 RPM, and the coolant temperatures is in the same calibratable high or low range at the time the diagnostic test last reported a failure, the MIL is enabled. At the end of the second trip, if similar conditions were never encountered, the saved conditions are updated to those most recently recorded.

Unique to the "misfire" diagnostic, is a special set of circumstances which may illuminate the MIL during the first "testfailed" reported to the Diagnostic Management System: If the misfire conditions are such that a potential exists to damage the Catalytic Converter as a result of high misfire levels, the Diagnostic Management System will command the MIL to "flash" at a 1 hertz rate during that ignition cycle.