Not to be rude, but I would take it to the dealer. It sounds like you cant fint it.

 

You are getting the CEL because of your Aftermarket Downpipe and High Flow Cat. I had the same thing happening to me. There is a very easy fix
for that. Get one of these: http://cgi.ebay.com/ebaymotors/CEL-F...spagenameZWDVW
and install it before your Post Cat O2 Sensor. That will get rid of your CEL guaranteed. The High flow cat flows too well and the O2 is interpreting
it as if the CAT isn't working properly. If you get the O2 out of the stream of the directly flow the O2 will read the CAT as functional.
I guarantee that this will work. Lots of people get that CEL after they install an aftermarket DP and High Flow Cat or Test pipe.
If you don't want to spend the money on the one from E-Bay you can get some Sparkplug Non-Foulers from Autozone. HELP Part number 42002.
Drill a 1/2 diameter hole in the first and leave the second in tact. Screw the O2 sensor into the one that you just drilled and then put the nondrilled
unit on top of that and then screw it into the bung after the cat. That cost you about 3 bucks and a little work drilling the hole and dremeling the rough
edges to make the O2 fit. That is what I have done and after I cleared the CEL it never came back.
So just do this, then clear the CEL and it will all be better. I promise !

 

Jake - I did take it to 3 dealers and they wont diagnose it due to mods. Aye I got desperate lol so no luck there. And Tom - Ill buy it and give it a shot. I turned the code off (Was for Pre cat o2 again) and ill see how long it takes to come back on. If I get that part and it does come back on though - im just gonna put it up for sale or trade it in. Ill go this weekend to our autozone as well and get the non fouler stuff. They're brand new too 800 miles on the plugs. Im just sick of seeing that damn orange light[:@] GTFO please.

 

So - As of 8-11-08 the pos is GONE. Traded it in for an 08 Nissan Frontier V6 4x4. After buying that part you suggested and having the CEL come on within 50 miles. F it. I am done with this vehicle. The amount of money I have put into it and the volume of people attempting to even diagnose it. Done. Dealers won't even help since it's modded which I think is a load of garbage imho. Soo Goodbye Audi. I have a ton of parts for sale now I will have up by next weekend. Ranging from my Motegi's w/ tires, Borla cat back, HKS SSQV, Turbo Timer, EGR items, MAF, OEM Airbox, Intake Spacer, ECS tuning OBDII Scanner, and a lot more.
Peace.