DTC Identification, Maturation, and Erasure

Once a test has been run, the Task Manager determines whether the system has passed or failed. It must then determine if the test has failed the specified number of times required to illuminate the Malfunction Indicator Lamp (MIL). If not, the Task Manager stores a maturing code. When this test is run again (on the next "trip"), the results are once again either pass or fail. If the component fails the test, a code "matures", and a DTC is set. The MIL is illuminated if an emission component is involved. If the component or system passes the test a specified number of times, the maturing code is erased.

NOTE: It is important to understand that a system need not fail a test under exactly the same conditions for a code to mature and trigger the MIL. It must, however, pass a test under the same conditions it failed the first time in order to begin the erasing procedure for the misfire and fuel system monitors.

Diagnostic trouble codes (DTC's) are erased if the MIL has been extinguished and the system or component passes the diagnostic test for 40 subsequent warm-up cycles. That is why it is best to attempt to diagnose intermittent problems soon after they occur. While a DTC may have been available initially, subsequent trips (that passed the diagnostic test) will have erased the code. The OBD II system, based on the latest inputs it receives, reconsiders its diagnosis for intermittent problems.

Of course, a diagnostic scan tool can be used to erase DTCs at any time. In addition, DTCs (along with records of successful trips) are erased whenever the vehicle's battery is disconnected.