o2 sensor on 2003 Hyundai Elantra

The o2 sensor will not go off. The car runs fine. Mechanics cannot find anything wrong with the car. How do we get the senor to go off?

Asked by for the 2003 Hyundai Elantra
Not sure what you mean, is the check engine light coming on? Is that what you can't get to "go off"? If so, can you post the codes that are being stored int eh engine module?