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 reset, I drive for about two days, sometimes one day 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.

 

Those mods could easily throw a code if you didn't get a proper weld around the entire pipe.