My 2002 Cadillac Deville "Check engine" light came on. What is there to check? The car appears to be running just fine.?

Start by taking the vehicle to a local auto parts store. Most stores will loan you the device that can scan the codes.

The code won't tell you exactly what's wrong, but it gives you a good idea of where to start looking.

You need to go to an auto store and have them read the codes that were set in your computer using their code reader. It might be a misfire or an emission leak.

Those "Check Engine" codes are more about the emissions system efficiency than any problem that would affect driving (although there's some overlap)

As another person said, take it to your local auto parts store to get the code read--for free! (a garage will charge you $100...a dealership, god only knows.) You can also get a code reader for fairly cheap--a CAN/OBD II will do the trick.

It's likely to be something simple--even leaving your gas cap loose can throw a code. It may go away on its own after 3 driving cycles, as well.

There is a sensor code to read. These codes indicate when unexpected conditions occur. Everything from low oil pressure to bad fuel air mix to a headlamp out.

Have a mechanic read it.

Appears?? When did you last change the oil with the proper Mobil 1 ?
Did you change the filter then as well? When did you top up the coolant?