the car still works perfect but the light is on is it just a o2 sensor

 

pull the code. to check the code look under the dash on the passenger side by the ECU. there will be a wire there with a female end with 2 openings... stick a paperclip or piece of short wire in both ends (making a connection from one side to the other). Once you do that with the car in the ON position the engine light will flash. Count the flashes.... a long flash = 10 a short flash = 1..... long long short = 21 short short short = 3... get the code and post it and i can help you from there.

Berz out.