Hey, i need some help guys. a while ago i got DC Jdm 4-1 headers, and a highflow cat put in. After a while i heard this rattling noise from the cat and i got it looked at. the guy at the shop said one of the screens in the cat was rattlign around and the o2 sensor wasnt reading right because it might have gotten damaged or the air wasnt clean enough. (he didnt realy explain it. my check engine light came on. i never had him fix it cuz i dont want to give him my business. (i just used him for an answer). anyways my car's exhaust has been smelling really bad lately and im guesing its running rich but i have no idea. is there anything i could do to see which o2 sensor is malfunctioning? or is there anything i could do to find out what the problem is? please let me kknow! thank you in advance