MIL Requests and History Codes

What happens when types A, B, C and D DTCs report failures and passes, and how the MIL responds. Includes the criteria for turning the MIL 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. Diagnostic tests are separated into different types:
- Code A is emissions related, and turns ON the MIL.
- Code B is emissions related, and turns ON the MIL if a fault is active for 2 consecutive driving cycles, except for any misfire and any fuel trim DTCs.
- Code C is non-emissions related, and does not turn ON the MIL, but may turn ON a service lamp if the vehicle is so equipped.
- Code D is non-emissions related and does not turn ON any lamp. Code D non-emissions related may not be utilized on certain vehicle applications.
^ When a type A diagnostic test reports a fault, 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 to have the MIL turn ON for that diagnostic test. Type C and D set the DTC but do not illuminate the MIL. The diagnostic executive has the option of turning the MIL OFF when 3 consecutive trips are recorded where the diagnostic system passes the test. In the case of misfire or fuel trim malfunctions, there is an additional requirement that the load conditions must be within 10 percent, the speed conditions must be within 375 RPM, and the coolant temperature must be in the same calibratable high or low range at the time the diagnostic test reported a failure. For a type C diagnostic test, the diagnostic executive will request that the auxiliary service lamp, if so equipped, be turned OFF at the next ignition key-on cycle or test passing report.

To clear a DTC for any diagnostic tests requires 40 subsequent warm-up cycles during which no diagnostic tests have reported a fail.