my converter recently starting clinking and clanking so i bought an aftermarket magnaflow cat. first i wanna know if anyones ever used an aftermarket cat and will it make my check engine light come on?? or is my sensor behind it.

also i called a muffler shop today and asked how much would it be to cut the old one off and weld the new one on there. he said $35 - $100 depending on if he has to take the whole muffler system off or if he can weld it while its on there. does anyone know if you have to take the whole muffler system off or not?