Will the check engine light come on?

pretty sure it will and youll lose a noticable lowend power from what ive read of other peoples experience. although you will gain top end. so it depends on what you use your car for. as for the cel light its most likely to happen unless i believe if u shim up your rear o2 sensor so its out of the way of the direct flow of the exhaust gas u should be all right.

No your undoubtably get one, belive me. Just stick a black insert behind the cel. You wont even know its there, then just scan once a week.

well i thought if u shimmed up the o2 sensor out of the way of the exhaust flow that it wouldnt set the cel light. also if u really understand electronics you can put like a resistor or some kinda electrical gismo into the path of the o2 that will trick the ecu into thinkin it is getting normal readings from the o2. this is called a o2 spoof. alot of other car companies make them when deleting a cat.

I've heard this as well and will post next week when I try it

my friend shimmed his o2 on a is300 and it didn't set the cel so i don't c y it would do if for audi then again germans man thought of everything lol

when i installed mine i got a cel. I installed the spark plug non fowlers and it went away. Check out this post, test pipe info (http://www.audiforums.com/m_39030/mpage_1/key_test%252Cpipe/tm.htm#39575)

I have a supersprint de-cat pipe and my car cel light has not come on.

However my A4 doesnt have a lamda sensor after the cat just before.

Cheers