Why does an engine light come on?
Check Engine ... self explanatory ... check the engine. Check fluids, air filter, water in the coolant system, oxygen sensor, bad spark plugs, electrical problems, loose nut behind the steering wheel - could be any number of things ... check everything in the engine area.
Check engine light will come on for various reasons. The o2 sensors could be one of them. If your belt breaks the light will come on and the engine will over heat. I mentioned the o2 sensor because when it comes on for this reason some people have had that light on for years without replacing the sensor.