Hey I'm getting these 2 codes in my audi A4 PO431,PO421,So do i hve to chage my O2 sensor or catalytic civerter, Plzzz help me !!!


Asked by Feb 07, 2015 at 10:58 PM about the 2003 Audi A4 3.0 Quattro

Question type: Maintenance & Repair

I'm keep getting these PO431,PO421
in my Audi,i gust last owner hve used
low octain gas that's y catalytic
civerter getting hot, So wht i hve to
do ?? :(

5 Answers


your car has a missfire. most likely a bad spark plug or coil pack. im guessing you have not much knowledge about cars so take it to a VW/Audi certified mechanic ONLY> never take your audi to the average joe blow mechanic unless you want more issues and many repair bills. Have them check your car for a misfire if you have not had a tune up done in a long while i would suggest having them do one. which usually includes new plugs and an oil change. and if your ussing low octane fuel in your audi your not supposed to You have to use only the best octane gas you can find since low octane gas will wreck havoc with your audis engine and emissions systems such as the catyletic converter.


So is there nothing wrong with o2 Sensor ?????


that would be a most likely no probly not but if the car persists after the tune up then start looking at a new cat. a new cat will run you up any where from 500-1500 $ depending on wher eyou buy it since they come as a full set built unit and should come with a o2 sensor if im correct.

Laura Gelles

I have an Audi A4 Quattro 3.0 2003 I went to the market last night and when I got back in car it wouldn't start. The dash lights went on, but no noise when turned the key. I was about out or out of gas. I thought it might be that. Got gas and put in car, still no start or noise. Today it's starting fine. Shows error code P0421 Any clues as to what is most likely wrong? Thanks so much!!


Sent a reply in private message be sure to check it and for separate issues you should open a separate tab. i follow all audi and vw questions so i should see the question within a days time thanks.

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