Originally Posted by Angel

All OBDII cars (1997 to present) have 2 O2 sensors - one upstream of the Cat conv. and one downstream. The ECU periodically checks the two readings to be sure that the cat is actually 'cleaning' the exhaust going through it. If you just delete the cat on an OBDII car (with a stock ECU...) then it'll throw a cade saying "Catalytic converter isnt working properly" - this would be a code that, if discovered, would cause you to fail most emissions checks.

And then about the tailpipe sniffs- ask the diesel guys how sleazy those are - when I had my OM617.951 and Ohio was sniffing it for Opacity - the limit was 20%, and my 'burned 1qt oil per 3k' read out at 3% - I could have been making clouds of black smoke and probably passed the emissions sniff. While gas cars have different tests (CO content, NOx's...etc..) they actual limits might not be very restrictive - this would cause some old/out of tune cars to pass a tailpipe sniff with flying colors. I must assume that the limits for tail-pip sniff failures are different from state to state.
I've heard of places/businesses that will re-flash an ECU to 'forget to check' the cat's, but I'm very sure that such a thing would be 'for off road use only'. I know that no one here would every illegally modify their engine's emissions controls.

=)

bottom line - you need to see how your state does emissions checks (Ohio used to to do both- OBD check AND tailpipe sniff) and plan accordingly. If the car being tested is 1996 or older, there is no way the state can check it's OBD computer for codes so its a sniff or nothing.

-John