Once the MIL is on, the Diagnostic Executive will turn the MIL off after three consecutive trips when test passed is recorded for the trouble code that originally caused the MIL to be activated.
If this situation occurs while the MIL is off the DTC that set when the emission-related fault occurred will be stored by the PCM in the Freeze Frame and Failure Records. The DTC will remain in memory until 40 warmup cycles (without a new fault) have been completed.
If the MIL was set due to either a Fuel system or Misfire related fault, there are other requirements to meet before the code can be cleared. Once all the requirements for these types of faults are met, the onboard diagnostics can validate that the emissions fault that caused the MIL to be activated has been corrected.
The additional requirements for a Fuel system or Misfire fault are:
Intermittent MIL -On- Conditions:
If the PCM detects a fault and then the fault goes away, the MIL will remain on until after three trips are completed without the same fault reoccurring. This type of MIL "on" condition could appear to be an intermittent fault, but most OBD II faults are not intermittent in nature.
An example of an intermittent MIL condition is described next. If the customer were to leave the fuel filler cap loose or off, and the vehicle was driven under the correct code conditions (meeting all enable criteria for an EVAP large leak trouble code), the PCM would turn the MIL on the second time it ran the EVAP Monitor and failed the test. The MIL would remain on until the vehicle was refueled and the customer tightened the fuel cap properly or noticed it was off. Once the vehicle was driven under the correct EVAP large leak code conditions, the EVAP Monitor would run the test. If the EVAP test passed for three consecutive trips, the PCM would turn off the MIL (this is true for some vehicles depending upon the model year).
However, in this case, the related trouble code would remain in memory until 40 OBD II warmup cycles occur (80 OBD II warmup cycles for Fuel system and Misfire faults) without the fault reoccurring. If the vehicle was brought in for service and a PCM Reset step was done, the MIL would then go out and the codes would clear.