The check engine light has turned on but no loss in performance.
Is this a problem with O2 sensor that needs to be replaced or do I really need to take it in for a check up?

 

There are lots of items that can cause a CEL. The only way to know what to change is to have the car correctly diagnosed and I do not mean with a handheld scanner or Autozone. The correct way to do this is to have it done with the MB Star system that should pinpoint exactly what the problem is. The others just give general info, no specifics. And, yes you should have it done as some items can lead to very expensive reopairs if not addressed promptly. It could also lead to an on the road breakdown and that will happen when and where you don't want it to.