2003 Audi A6 P0421 Diagnosis

Can somebody help me further diagnose? The car seems to run fine, but I'm a little uneasy to keep using it with the Check Engine light on.

The code is P0421.

My car is a 2003 Audi A6 Quattro 3.0

yeah your not really hurting anything by driving with that code...other then the enviroment lol it just means catlytic convertor isnt cleaning up the exhaust like it should... check with a dealer on getting cat replaced i think they have a 8 year 80k mile replacement free thing they have to stand behind

Replacing the catalytic converter doesnt always help. I have been having the same issue...the P0421 code in my 04 eclipse. I have replaced the O2 sensors (which they recommend before the cat) PCV valve, plugs and plug wires (which is all the things they suggest fixing prior to getting a new catalytic converter) and had no luck with that so I have now replaced the converter and the light is back on (only 100 miles after replacing.

As for the length of the warranty, that is completely dealer specific as mine was only 3 year so I had to pay to get mine replaced.

I am in no way doubting what you are saying, Im just letting everyone know that replacing the cat doesnt always fix the problem.

If you have any suggestions for me please let me know. Im to a point now of wanting to use a stick of dynamite to repair it

Jenn, most times replacing the cat and/or sensor does the trick. Did you use quality OEM Mitsubishi parts or aftermarket parts? If it's aftermarket parts that's probably why it failed so quickly.

I used the universal O2 sensors, and took it to an exhaust shop to have the cat done...after it was finished I notiched that what ever he cut out and charged me for was something that was completely BEHIND the second o2 sensor....how can it pick up a reading on something after it? Im beginning to believe that I got SCREWED! He also told me a few things (that i dont wish to disclose on here) but said he could do something and then by pass the 2nd o2 sensor by relocating it to throw it off....Im sure you know where im getting at......


If I go and get direct fit Cat and non universal o2 sensors should that change it?