What causes a Ford Fiesta's check engine light to come on?

My Ford Fiesta's check engine light recently came on and I'm not sure what the issue is. The car seems to be running fine, but I'm worried there may be a problem under the hood. Can anyone offer some advice on what could be causing this?

The check engine light could indicate a variety of issues, such as a loose gas cap, faulty oxygen sensor, or something more serious. It's best to get your car checked by a mechanic to determine the exact cause.

I had the same problem with my Fiesta and it turned out to be a faulty sensor. I replaced it and the light hasn't come back on since.