The check engine light is there to indicate an emission related problem. It is conected to the on board diagnostic system in your car. In some cases it will come on due to a mechanical problem, provided the problem causes emission faults. There are in excess of 80 fault codes relating to emission problems. The car needs to be diagnosed properly to find the cause of the light. Some problems include oxygen sensor failure, ignition missfire, vacuum leaks, etc. It can also be triggered by an automatic trans problem.