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i think you are barking at the wrong tree.
simple logic would suggest if your cat is bad it would not be smart enough to discriminate between hi nox and low hc ... et al.
a bad cat is a bad cat and makes a mess of all the gases coming into it.
hi nox is traditionally as you rightly observe caused by air leaks some where upstream of the cat. it can also be aggravated by a failing o2 sensor sending back the wrong signals to the ecu.
how old is your o2 and when was it last checked for accuracy?
have you really checked for vacuum leaks?
arbitrarily changing cats is not the same as actually determining what is the cause. and they are not cheap to buy to boot.
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