check engine light is on

Could be a lot of things. There is no way to know with only that information.

Is the car running well? Have you detected any issues with performance?

Take your car to your local Auto Zone. They will put their hand-held scanner on it and give you the error code for free. Also, they can clear the code to see if the light comes back on.

My check engine light was on once, and I asked a mechanic why he thought it was on. He asked me if I used name-brand gas or cheap gas in my car. I told him that I often ran cheap gas. He told me that that could be the reason. He said that he always uses name-brand gas, and his check engine light never comes on. He told me that my O2 sensors were probably coated with contaminants from the cheap gas, and that was likely the reason for the check engine light being on. Since that time, I have almost always bought name brand gas; I buy the cheap gas only when I'm about to run out, and that is all that is available.

One time we had a Volvo. The check engine light came on; the local Volvo dealer told me to replace the gas cap; I did, and that cleared the check engine error! In that case, the gas cap apparently wasn't making a good enough seal, allowing vapor to leak out into the atmosphere, triggering the check engine light.

Only a skilled mechanic, with a code reader, can tell you what is really going on. But I can tell you that some mechanics have very advanced computers, and they can tell you EXACTLY what is the problem.