I can tell you that it's against federal law to tamper with any of your cats. I think after your car has somthing like 60K miles, you can replace the cats with new ones if they've gone bad.
But, you're right, testing is done by the state. For instance, the sniffer test they do in CA is all mandated by the state.
Keeping the stock 3rd cat, I know of a couple guys who have passed PA sniffer testing.
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Slow and low, that is the tempo.
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