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it just brings an an error code if anyhting is wrong with cats

if the rear o2 reads diff to the front o2 within a range it brings up a code

u cannot make or import a car in US if it does not have this feature

and afew other features

i got asked to try come up with something to make older efi cars meet new usa standards so someone here can export nissan skylines to usa but they had to meet this and afgew other things but was goin to be tooo expensive

usa cars also have fuel tank pressure sensors and other stuff thats compulsary if u want to sell a car in usa