Why check engine is on

The information below was complied by me, it explains what causes the check engine light to come on and the process of finding the cause.

When the Check Engine light (or Service Engine Soon or small picture of an engine) is turned on, the systems computer control unit (called the PCM) has sensed a fault in the electronic controls of the engine and/or the emission system. When this happens you will seldom feel the car engine run any different. There are about 200 possible system faults (the gas cap is just one of them) that turn the Check Engine light on, to know what has developed a problem you must have the system tested, this is called an OBD2 fault code test, Kragen/Oriley and Autozone parts stores (USA only) do this testing for free.