The most common cause for CEL is evaporative leak detected, usually someone forgot to tighten their gas cap, but it can be a multitude of reasons. Generally if the light is on constant it is just some error code, but if it is flashing it is more serious. Sometimes it is the sensor itself that has gone bad. With the mileage you are mentioning, I'll almost bet it is an O2 sensor gone bad.
If you go to an Auto Zone or similar store they will read the code for free. Also check your owner's manual for a method you can read the code, usually by turning the key on and off five times in a row or something like that, and then look up the code on an online web service...