my car idles rough so i changed the plugs thinking they were bad but it still does it. I went to advance auto and they put it on the OBD2 computer and P0420 popped up...i believe it is catalist something ineficiency...the guy said it was my o2 sensor...is that right or is there anyone have any other ideas on what it could be.

 

*Moved to Problems and Service, please post in the correct section*

 

throttle body.... go search, loads of people have this issue