I replaced both of mine because the exhaust manifolds were leaking and it overheated the cats(lean condition or something) Not that its pertinate here, but I couldn't afford new cats(that section of the pipe with the two cats is almost $1400). Called Magnaflow and got 2.5", OBDII cats that work just fine and only cost $72 each from Jegs. Had to have the rest of the exhaust replaced, so while it was apart, had those installed. Its what I did, yours is up to you. I really have to agree with Tech above, both don't go bad together. So its either "factory" or aftermarket. Bob