There are fuel additives that claim to clean the catalytic converter but I would be skeptical of such claims. I suspect they are mostly just fuel system cleaners. Once a cat goes bad all you can do is replace it.
There is a product called "Cataclean". Ask for it at your local NAPA auto parts store. I've never used it myself but one of our customers has, a couple of times. He claims that it works. If you have a Northstar engine, confirm that the head gaskets aren't leaking. Coolant will kill a catalytic converter. HTH. -Jim
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