MIL Illumination

As described previously, the Powertrain Control Module (PCM) Task Manager is responsible for operation of the Malfunction Indicator Lamp (MIL). This is an important function, since unnecessary operation of this lamp has a negative effect on customer satisfaction. In order for the MIL to be extinguished, the most recent malfunction must pass diagnostics on three consecutive trips after the MIL has been illuminated if the MIL was illuminated from one of the major monitors.

If a Diagnostic Trouble Code (DTC) is stored due to misfire or fuel system related problem, the PCM requires that the engine return to the operating condition at which the trouble originated and be diagnosed once again. The PCM must perform this task and pass three consecutive times to extinguish the MIL. This task may be quite difficult to perform, for example: If the problem initially appeared at wide open throttle (WOT) and high RPM, the customer may not revisit that driving condition for quite some time. Because the PCM has not had the opportunity to retest under those conditions, the MIL remains illuminated long after the concern has been eliminated. Even if another malfunction occurred after the misfire or fuel related problem, the PCM reverts to the procedure to extinguish the MIL for misfire and fuel monitor.

If the vehicle Is driven while the engine Is misfiring, and the misfire is severe enough to cause catalyst damage, the MIL will flash. If the condition that caused the sever misfire disappears, the MIL ceases to flash but remains illuminated until the criteria has been met that can cause the MIL to be extinguished for misfire or fuel monitors. Continued vehicle operation while the MIL is flashing is not recommended.

Finally, if a comprehensive monitor caused the MIL to be illuminated, the PCM must first recognize that the engine has been run for at least two minutes then, pass or abort the O2 and EGR monitor before it can verify that a previously malfunctioning component is meeting the normal operating conditions of that component. Also, the secondary air injection monitor must have either run or aborted before the component can be verified.

Anytime the MIL is illuminated, a DTC is stored. The DTC can self erase only when the MIL has been extinguished. Once the MIL is extinguished, the PCM must pass the diagnostic test for the most recent DTC for 40 warm-up cycles. A warm-up cycle can best be described by the following:

^ The engine must be running
^ A rise of 40°F must occur from the time when the engine was started
^ Engine coolant temperature must reach at least 160°F

Once the above conditions occur, the PCM is considered to have passed a warm-up cycle. Do to the conditions required to extinguish the MIL and erase the DTC, it is most important that after a repair has been made to erase all DTC and then verify the repair.

As in the past, the MIL can be used to produce diagnostic trouble codes. Quickly turning the key to the RUN position three times allows the MIL to flash out any existing flash codes.