[Treeve]

Cats don't cause NOx, high combustion temperatures cause NOx. Running lean will do this.

Best cats are OEM spec for your engine, so OEM commodore ones would be a good first choice, second on the list are XR6T cats, and finally, suitably sized ceramic core 400 cpsi (cells per square inch) cats from ebay and the like. Don't go for second hand - once a car has reached 160,000km it is considered that the cats would be fully depleted / damaged beyond useful life.

Incorrect fueling will damage cats very quickly, so as I've mentioned a few times in the past to pass emissions you need 14.7:1 for a cat to function effectively, and also the rapid fluctuation from rich to lean offered by a narrow band sensor. This replenishes the cat of both oxygen and fuel to aid the combustion and conversion process.

Treeve