So for a while my 05 Grand Prix has the check engine light on. Today I went out and had the codes read. I'm getting P0133,P0420,P1133,P1134. I've these problems before and had a shop replace the O2 sensor twice. I'm tired of paying him the same thing every time to have this continue to happen. I want to fix it myself if possible and try and eliminate this problem. I don't know if it matters but the car doesn't get great gas mileage either. Any help is appreciated.