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Error codes P0421 & P0431

talbius Dec 8, 2018 at 6:59 PM

  1. talbius

    talbius New Member

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    Hey guys

    Need some help. New Audi S5 owner here (2008, 4.2 V8, 110K, FASH). Love the car but having trouble with vehicle emission light coming on within 3 days of having it.

    Error codes are showing as

    P0421 Warm Up Catalyst Efficiency Below Threshold (bank1)

    P0431 Warm Up Catalyst Efficiency Below Threshold (bank2)

    The dealer had a new mot put on it before I picked it up, it was mot'd with a local garage.

    After having the error codes, I took the car to an Audi specialist who said the sensors are all fine and basically both cats are knackered and quoted roughly £1500 parts and 15 hours labour for both banks.

    I informed the dealer who sold me it and he wanted the garage who mot'd it to take a 2nd look as it passed emissons during mot test literally a few weeks earlier.

    They done further emissions tests on each cat which they say passed, im told the dealer then sent an audi technician who apparently said the cats are functioning ok. Its due a service in 3 months and they suggested changing the spark plugs then to see if that helps.

    With conflicting information im really confused. The car feels and drives fine. No history of a carbon clean.

    Can anyone give any advise? Anyone had something similar happen before?

    Thanks

    Talbius
     
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  3. Jps3290

    Jps3290 Active Member

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    The errors codes are due to cats being below 100% efficiency but will still pass an MOT.

    You can buy a post cat O2 sensor extender for about £9 for each cat to stop the error. I had to do it with bank 2.of mine and no more errors.

    If u need a link to item.let me know.

    Sent from my SM-G930F using Tapatalk
     
    voorhees likes this.
  4. talbius

    talbius New Member

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    Yeah can you send me the link please. Thanks JPS

    How long have you had the sensor extenders fitted?
     
  5. Jps3290

    Jps3290 Active Member

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    I only got 1 on Bank2 post cat sensor as my other one hasn't flagged up any errors. I did try changing sensor on bank 2 thinking sensor was knackered but it wasn't.

    Everytime I reset the error the MIL light was coming back literally every 30 to 40 miles.

    Since adding the extension I've done just over 500 miles and been spot on ever since.

    This is for my RS5 but message them to ensure its same size for your car. Or check size of lambda sensor online which should mention the thread size. Pretty sure it's same part though. These are good quality items. Took me 20 mins to fit from start to finish as in my RS5 I had to remove one of the throttle bodies and intake boxes to reach and still very fiddly but you will manage. It a cheap fix.

    Look at this on eBay
    https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.co.uk/ulk/itm/282715300794

    Sent from my SM-G930F using Tapatalk
     
  6. Jps3290

    Jps3290 Active Member

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    Your not getting any other errors are you like fuel trim too lean or upper limit exceeded? Just the ones you have mentioned? If you getting these extra ones it's more likely your MAF.

    Sent from my SM-G930F using Tapatalk
     

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