i've got a d16y8 running standard obd2 setup. i have an apexi ws 2 exhaust but recently had the piping resized to 2.75" for a future turbo app. i had the apexi silencer inline welded and now i'm throwing a check engine code of cat. below functioning threshold or something like that. seems to be affecting gas mileage also. so could it just be a bad o2 sensor or what? help!!!!

 

come on need some help!