Hi Guys! I'm new for this forum, but need your help.
Got 2001 S4 couple days ago, 125K miles. I know it's crazy, but love this car!
Anyway, after a few days of driving the CEL came on. I went to local mechanic and got 3 codes: P0421, P1127, P1129. He changed the oil and air filter, reset CEL and after 95 miles it got back again.
I did a lot of search on this forum and didn't find any solution that really fixed the problem, except replace cat. Even that not always a cure.
I'll go to read codes again today and see which one I get this time, but I'm pretty sure P0421 will be one of them!
Anyway, any suggestions appreciated, what should I do first? Thanks in advance!

Update: As I sad, after 95 mostly freeway miles CEL came back - system throw only P0421, as I expected. Than, I after reset and another 45 (in this case - city only) miles CEL on again, but this time no P0421 code (checked after 85 miles), only P1127, P1129.
The guy who sold me the car suggested It needs MAF sensor to be replaced. Hate to waste $200+ on just a suggestion. Can that be it? Any good explanation? Any thoughts?

 

Interesting thing happened this morning - I've started my car - CEL was gone, so I drove for a while and was even able to got state sticker. After next 10-15 min CEL - back again. Going to check the codes later today.

 

not the maf.... system too rich... it's most likely your O2 sensor befor or after the cat.

 

you need to text voltage.... or just replace them... no more than 100 each for oem from ebay...

 

I have the same problem with my 01 S4. I replaced the 2 front O2 sensors (I am under the impression the S4 has 3 O2 sensors?) and can't seem to pass emissions. I'm about to pull my hair out. I've had the spark plugs replaced, vacuum line leaks fixed, coolant temp sensor replaced and still have the P1127 & P1129 codes! I also have the 0300 Code. Help! Any ideas? I am so sick of throwing money at this car.

 

There are 4 O2 sensors on the car.....two on each exhaust one before and after each cat.
I want to see what those codes mean...will have to check back later..on the go rght now

 

Well, I got i fixed a while ago, so here is my summary:
- both bypass valves and hoses were leaking
- spider hose was leaking
- N75 Valve dead
- some hoses under Y-pipe were crashed (Do NOT apply exceed force trying to install y-pipe)

Hope it will help you - 2.7T lovers.
-Good luck

 

Hey titanko - so does your S4 run great now? I am at my wits end with this car! I guess I'll take what you had done to my mechanic and see what he says. How much was all that, do you recall??

THANK YOU!

 

Well, it was running great for a few months until I got hit by the truck. Total loss.

Whole deal cost me around $500-600 with labor at local mechanic.

BTW,
Before finding the right guy, I've replaces both EGT sensors & Intake air duck but as turned out they were fine.

My advice - ask on this or other Audi forums to recommend you good mechanic for S4 or 2.7T in your area. That did it for me. Until I found the right mechanic - every 5-6 shops I went with my car were shooting dark, try this, try that....

Good luck!