Hi if you are getting noxious fumes into the cabin you have bigger problems than just a CAT replacement. Take it to a machanic.
Fumes are either coming in through the aircon or some other path - are they exhaust fumes?
First question is what kind of fumes are they? Second question is how are they getting into the cabin?
Either way you should not have noxious fumes entering the cabin abd it sounds like a serious risk. Get it checked by a professional would be my advice.
re: CAT
there are two - one in the downpipe and one after the downpipe. In some states it is legal to remove the second and replace with a straight pipe. "Race pipe" "Mid pipe" "Test pipe"
Replacing primary CAT means replacing the downpipe.
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