My check engine light has come on so I had it checked at autozone and another place to get a second opinion and the code that has been triggered was the P0420. From doing research it seems like the most likely problem is the Cat or the o2 sensor. From anyones experience is it usually the Cat or the o2 or maybe both? I guess I will start by replacing one and then the other if need be. Also how much is my gas mileage being affected? I can tell a little bit but I have only had the car two days before the check engine light had come on.

 

I have the exact same problem. I wouldnt worry about it if you dont have to, it shouldnt effect your driving, its emissions.

 

P0420 is normally the After cat O2 sensor. Change that first since is a bit cheaper.