Check Engine Light w/ P0420 code
 

Good morning guys,
I'm having some major difficulty with my 6 speed 2004 Audi A4 1.8T. About two months ago, my check engine light came on driving home from work. I had the code read and it came back p0420. Under inspection. I found that I had a small crack in my resonator so I had it removed and had straight pipe welded in place. The fix appeared to work until the bracket for the flex pipe rusted off as well. I had that replaced with a straight piece of pipe as well. Since those two fixes. I cannot get my check engine light to come off with the same code still coming up p0420. Since the two exhaust fixes, I have replaced the catalytic converter as well as both the upstream and downstream o2 sensors. My question is this; would altering the exhaust the way I have, continuously effect the check engine light ? After having it reser, i drive for about two days and then it comes right back on. The cat I purchased was a direct fit catalytic converter and the o2 sensors were purchased directly from Audi ? Thank you for any help guys. Greatly appreciated.

The exhaust fixes SHOULDN'T have any effect on the catalyst efficiency, provided there are no leaks. A leak upstream or immediatley downstream of the cat could set the code since false air (oxygen) can get sucked into the exhaust and skew the sensors. Since there are no other codes, the engine is most likely operating correctly BUT it would be a good idea to hook up a scanner that can read real-time data and monitor the upstream O2 for correct switching and also, to be sure the other inputs and outputs are in tolerance. If too much goes in, the cat won't be able to do it's job and too much comes out,,,,,,,,,,,if you get what I'm trying to say. Can't decide how to describe it. The cat can only catylalize so many pollutants at one time.
For a catalyst (or evap) code to set, the car has to go through two drive cycles where all monitors are run and it "sees" the same fault both times.

There was a reflash for the ECM on some models and years to address a false P0420 (and P0430 on V-type engines). Might be worth checking into. Basically, the software was way too sensitive and would set a fault when none existed.

Aspen79,
Thank you very much for the response. I am pretty sure I understand your explanation about the catalytic converter as well as how it catalyzes the pollutants running through the system. I also have an additional question. Even though the catalytic converter is brand new and a direct fit from 1AAuto, could the system not coincide with the existing o2 sensors which is causing my check engine light to remain on ? Also, before I removed the original catalytic converter, the check engine light would periodically go off on its own but only if I parked my car outside. When I kept it inside the garage, It would never go out. I'm completely stumped on what my next move should be. I personally feel I may need a higher quality catalytic converter (magna flow, borla, etc) but I'm not sure at this point. Again, I greatly appreciate the help !!
Bruce