sjames

Will it give me any sort of check engine light error? How much gain should I expect seeing im chipped with exhaust. Thanks.

RVTS4

I would guess yes and if you get caught by the law its a big fine...In MD we have 2 year emissions testing so at least in my state you would have to put them on for inspection. cat on/catoff

rao

You will get the code for failed cats from the second O2 sensor on both banks.

Drozd

Here is another thought, done on a GMC Typhoon.
In Colorado, what they do is look underneath for a cat. If they see one, OK. If the emission test is within specs, no problem. Remove cat, cut open on TOP, remove guts, weld shut, remount. Weld is against floor pan. Seems like modern engine management systems control emissions quite well, and these days cats are political, more than engineering necessities. The Typhoon shows up with a slightly higher spike than with the cat, but well within specs. Been this way thru 4 test cycles. HOWEVER, it's a technical violation, so I'm not promoting it here. Just another option.

DarinS4Turbo

You put in a circuit to fool the computer into thinking the cats are OK. Without the cats, the rear set of O2 sensors will oscillate just like the front set, centered around 0.45 volts (from 0-0.9 volts, give or take). This tells the computer the cat isn't doing it's job. Normally WITH the cat, the rear O2 sensor would oscillate around a lower voltage (say from 0-0.4) due to less oxygen. You can wire in a simple circuit to mimic this characteristic, and the computer will be happy. I think Mustang owners have down this with off-road y-pipes (and there is a place that sells the connector/circuit all ready to plug in).

rao

Good luck trying to fool the S4's ecu - it is possible but I don't think anyone hs tried