Test Sequence

What constitutes a particular On Board Diagnostics (OBD) test is important because, in many instances, the vehicle must fail a test more than once before the Malfunction Indicator Lamp (MIL) is illuminated and a diagnostic trouble code (DTC) is recorded. Tests that illuminate the MIL when a single failure is recorded are known as "one trip" monitors. "Two trip" monitors allow the system to double check itself and help prevent unnecessary MIL illumination.

If the conditions to run the test are not met on consecutive key cycles, the information from the first test is not lost. The Task Manager waits until the next time the appropriate test conditions occur, and then continues the count.

There are times when a test is held up, pending the resolution of a related problem as indicated by the MIL. Testing the system or component at this time guarantees it will fail erroneously, so the Task Manager doesn't bother.

At times there are other tests running or existing faults that conflict with the operation of a test. In that case, the Task Manager chooses not to run the test. No trip is completed.

A test may run, yet have its results suspended until the results from another monitor are received. Once that occurs, a test is completed.

If this is a "two trip fault" and the test failed the first time, a malfunction on the second consecutive time the test is run (even if there were key cycles between the tests) will illuminate the MIL. If the malfunction does not occur the second time the test is run, the MIL does not illuminate, and no DTC is recorded.

The Task Manager can turn off the MIL if it records three consecutive trips when the system passes the test. However, with the misfire and fuel monitors, the system must past the test under the same circumstances (within 375 RPM and within 10% of the load) under which it failed the test. More information on this is presented in the sections on DTC maturation and freeze frame that follow.

In summary, it is the task manager's job to determine if the conditions are appropriate for a test to be run, know the definition of a trip for each test, and record the results of each test when it is run.