Yeah, please do. I just know this from previous posts. Try doing a search about the code to see if you can find anything else. There's a lot of threads with this number so good luck.

 

Alright.

I'm going to order the O2 sensor. We'll see what happened after I put it on. I'll let you guys know.

 

I dont think its the sensor or the cat, im having the same code on my car. I bought an o2 sensor and decided to hook up the car to the snap-on modus diag tool at school (uti) and i saw that all sensors are working propertly in realtime. My car feels fine and no funny smell from the exhaust. Like you im trying to solve this puzzle but it seems like everyone runs out and changes a cat or sensor just to see the code return.

In my search ive moved onto the combi valves. It helps with cat warm up at start up and sometimes idle. Im going to check all the lines and even the valves themselves. If that doesnt work im going to check the wastegate funtions. those might through off the sensor readings if they are sticking open or possibly close. I am trying to focus my career on diag. so i hope this helps you and stops you from spending cash when you dont have to like so many other do and have.

 

Replaced O2 sensor last week and no more check engine light.

 

cool, but if it comes back check this out.
The scan tool said my sensors are all working fine, so i dont want to change em.

This is the post i found:

Best Answer - Chosen by Asker
most likely, the first 2 codes are caused by the condition that is triggering the third. when your secondary air is performing incorrectly, it causes your veh to run rich on startup, so the comp thinks your converters aren't operaqting correctly. my guess is that you have a 2.8l V6 from the codes. most likely, you have a torn vacuum line going to the kombi valves on the back of the heads, not allowing the secondary air pump to perform correctly. your kombi valves are aluminum cylinder looking things on the back of each head. look at the vacuum line that goes to both vlaves and try to find a leak. it's pretty common. hope that helps
Source(s):
7 years as an audi tech
3 years ago
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Asker's Rating: Asker's Comment: very helpful in the location where I can start to look at. Is this what you are searching for?Rating: Good Answer Rating: Bad Answer

http://answers.yahoo.com/question/in...0104900AA09JlR

 

That seems like the answer of a 1990's audi guy... The truth is that he's correct, the 2.8 and 3.0 have similar codes when it comes to low efficiency bank one, P0421 for bank 1 and P0420 for bank 2 (or maybe is viceversa?) , but, the bad thing is that your VAGcom tool just reads you heat and RPM expectancies... a common law, the I'm completely broke law reads to go and get the cheapes fix first, it might be that solution, but if it is not, then the other more expensive solution is called for... In this case the O2 sensor is the cheapest solution, for about 200 bucks, if not, you'll find yourself replacing the whole catalytic converter for about 800 bucks...

 

As of today, check engine light has benn good to me so far.

 

I know this is a little late, but your cats if you are having no symptoms are probably fine. Two TSB's were announced by Audi for the C5 A6 cats back in 2005 - your ECU just needs a reflash for the updated thresholds. Without the reflash, sometimes you can throw codes such as P0421 as the Audi Tech explained about the cats @ warm up from cacaro's post.

http://www.allworldauto.com/tsbs/NHT...tin_50360.html

http://www.allworldauto.com/tsbs/NHT...tin_79227.html

 

Hi John,

Just bought a 2001 a4 1.8t. It is throwing a p0420 code. So I've been searching and found your thread.

How's the car doing? Is the cel staying off?

Any lessons learned that you can share?

thanks!
Jim

 

Sorry for the late reply guys.

The code IS BACK. I'll try to check the vacuum lines behind each head for the Kombi Valves, and if that doesn't work, I'll check the wastegate.
Jim-- Do you think you can point out to me where this wastegate is located?

Appreciate it!