QUESTION: I heard that a loose gas cap can trigger the light. Why is that?
ANSWER: Because fuel systems on late-model vehicles are sealed to prevent the escape of fuel vapors into the atmosphere. If the gas cap isn’t tightened all the way, or the cap leaks because of a damaged seal, OBD II will catch the fault, set a code and turn on the MIL.
QUESTION: My Jeep will misfire sometimes, but my Check Engine Light doesn’t come on. Is something wrong with it or could it be burned out?
ANSWER: Misfires can cause a huge increase in hydrocarbon emissions as well a loss of
performance, rough idle and other problems such as converter damage if the misfire is steady rather than intermittent.
Misfires can be caused by a lot of things including worn or fouled spark plugs, bad plug wires or ignition coils, dirty fuel injectors, air leaks or other conditions that cause the air/fuel mixture to run leaner than normal, or even a loss of compression in one or more cylinders because of a burned valve or blown head gasket.
All engines will experience an occasional misfire, but if the misfire rate exceeds a certain threshold, OBD II sees it as a problem, sets a code and turns on the light. Sometimes, the MIL will flash while the vehicle is being driven. This is a warning to the driver that the engine is misfiring under load.
QUESTION: I have to get my truck through an emissions test. Can’t I just reset the CEL by disconnecting the battery?
ANSWER: One very important point to keep in mind with respect to the Check Engine Light on OBD II vehicles is that you must use a code reader, scan tool or scanner software to read and clear codes. There are no manual flash codes on OBD II vehicles.
Disconnecting the battery in an attempt to erase a code so the CEL/MIL will go off is also a very bad idea because it can cause the PCM to forget learned settings. This may cause additional driveability problems, and may even require using a scan tool to “reset” lost information.
