On a vehicle with a four converter system and only one cat is bad, why cant you just replace the cat that is bad so long you dont interfere with the location of the o2 sensors? I heard from this supposidly owner of an exhaust shop for 25 years that this is not possible and tampering. My question is why have so many people done it successfully without comebacks from the CEL and why would they even make universal cats vehicle specific if they were "illegal"? I am starting to think that my source of owning an exhaust shop is not educated on this matter.