You can try & replaced the o/2 sensors first & see what happens for they will need to be replaced anyway. Just use factory type with factory connectors , no universal!

 

Hmmmm, that sounds like tossing parts at a problem without being sure....in all my other cars, 02 sensors throw their own code AND the car starts driving like crap and gets horrible mileage as well....not sure that is what we need to do right now. Any real Audi techs on here. Someone that works on these things for a living???

 

16804/P0420/001056 - Ross-Tech Wiki
Your car did throw a code. You just did supply us with it. If its a p0420 its a bad cat. I forgot the individual code for the 02 sensor. Either way catalyst is normally a cat. Take it to autozone ask them to print out your thrown codes. Should be numbers along side of them.

 

Ive been dealing with the p0420 code for a year! changed both o2 senors and tried to put in paint thinner thinking it would clean out the converter but no luck! I decided to try the spacer from 034 motorsports, installed it and the code is gone with no engine light for 4000 miles now. So before you change out the CAT, try this first before blowing $1300 bucks. No that I know its the CAT, i will be saving my pennies to get it fixed right!

 

If you live in a state that is not strict on emmsions you can get a testpipe or a high flow cat. Instead of forking over 1300 u can pay 300 for a hfc or under 200 for a testpipe. Plus u gain power.

 

I remember having a GM (88 Jimmy) and it wont throw a code until the CAT was totally stopped up!
Ive read somewhere that when our CATS (VW/Audi) drops below 95% efficientcy, it throws the "below threshold" codes (P0420 and the like) The o2 spacer is only about $20 bucks shipped. It was a very simple install, (minus the tight spot to get to it!) only took me about an hour. The only other way to find out what is going on is to get a Vag-com and read the sensors! If you try to use a cheap converter the light WILL come back on! From research and experiance, the factory CAT is the only way to go unless somebody else proves me wrong. I just had my car inspected and it passed with flying colors (sweating at the same time)!

 

Thanks for the tips...the funny thing is, the code came right after putting in some Fuel injector cleaner and hi pressure washing the engine bay...I thought I may have damaged something in there...but after resetting the code at Autozone, it comes back after about 100 miles....Florida is a no emissions state, if I get cat bypass/test ipes installed, doesn't that trip a check engine light as well? This thing runs great, and I bet utll run even better with those cats gone or gutted....anyone done that on the 3 liters yet? Need some feedback...I am NOT pay what Audi wants for a cat...no way in hell!

 

hfc with a spacer on the 02 will rid the cell most of the time. Or if u get the car chipped the programmer should be able to write out the 02 code.

 

OK, plz forgive my ignirance, but what is an HFC, and what kind of spacer? I understand flashing a cars logic module and all...just want this thing done...that light is the burr under my saddle! I read one guys article about how he took his Cats off, basically just scrubbed the inlet screens, and it worked...others just bypess/remove them ( I'd love to do this, but am worried that something crazy may happen....?) Some cars are just so very dependent on Cats and their related components, so knwing German cars like I do it makes me a tad nervous...really cool/nice car...just want it fixed WITHOUT dropping big oennies on new Cats....maybe Audi will do a post 80,000 mile warranty repair. I know BMW would....

 

HFC= High Flow Cat

The spacer trick uses a couple of spark plug anti-foulers from Autozone, or the like, one drilled to fit the down stream O2 sensor, screwed together, then installed in the sensor bung. It tricks the computer into thinking the HFC, TP, or below threshold cat is working properly. Does it work on the 3.0L? I don't know, but it's a pretty common trick for the 1.8T.

Audi is notorious for having extremely high tolerances on the cats, and I believe they have acknowldged it, and will "re-flash" the ECU for free at the dealer to allow for more tolerable cat standards.