Why does the check engine light remain on with no performance problems? on 2001 Hyundai Elantra

I had my exhaust system replaced and my O-2 sensor replaced in the exhaust pipe.

Asked by for the 2001 Hyundai Elantra
There is most likely another problem with the car that is turning the check engine light on. We see more functioning 02 sensors repalced than any other part on a car.I would suggest you have someone diagnose the problem