DTC and MIL Operation

OBD II regulations include carefully defined minimum requirements for actions relating to fault detection, including:

^ How quickly the on-board system must be able to detect each type of failure, set a DTC and turn on the MIL.
^ Under what conditions the On-board system may turn off the MIL for a fault which happened in the past but is no longer present.
^ Under what conditions the on-board system may erase stored DTCs for a fault which is no longer present.
^ The categories of scan tool information which must be available to technicians.

Illuminating the MIL and Setting DTCs
Unlike OBD I which had one basic procedure for turning the MIL on & off for any major fault, OBD II employs several different strategies. These will be covered in detail later in the book but here are the major points.

^ A serious misfire problem which could result in catalyst damage will turn on the MIL and set a DTC as soon as the fault is detected. The MIL will flash on & oft until the severity of the misfire is reduced to the point where the catalyst is no longer in danger. At that point the MIL will stay on steadily.
^ All other faults must occur during a second driving cycle before they will turn on the MIL and set a DTC.
^ The MIL will be triggered by a fault in the 900 automatic transmission which would increase emissions above allowable limits. The TCM requests that the ECM turn on the MIL. (The "Check Gearbox" light will also be on).