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Thread: Ford Fiesta P0420 code problem

  1. #1
    Join Date
    Apr 2011
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    Default Ford Fiesta P0420 code problem

    Hi,

    car: Ford Fiesta 2004 1.4i Zetec engine, current mileage 45,000,
    fault: p0420 error code

    First time poster and was wondering if any one can help. Have read through the forums and have got a lot of useful info on p0420 code which is "Catalyst System Efficiency Below Threshold (Bank 1)".

    I know that this can be a number of different causes to this code. I have run an OBDii diagnostic check with a scanner hooked up to my laptop and have recorded the O2 sensor voltage output. I warmed the car up by driving it for 20 minutes and then hooked it up. From what I can tell the voltage of the front O2 sensor fluctuates between 0.1 and 0.9v and responds to throttle almost instantly. The rear 02 sensor voltage seems a lot flatter on the graph with smaller fluctuations.

    I have attached two graphs one of the car idling at 750 rpm and the other of the car at 1500 going up to 2500rpm. One each picture the 02 sensor voltage is shown by the bottom graphs with the blue line being the front 02 sensor and the yellow being the rear.

    On the first graph, when it is idling, the time between the front O2 sensor reading a high voltage (~0.9v) and then it dropping down and reading a high voltage again is about 3-4 secs.

    If any one can tell me whether they think there is any problems with the O2 sensors I would really appreciate it. If you need any more info please ask. Thanks very much for your time.

    o2 sensor at idle
    http://www.screencast.com/users/JonS...8-f772ce3daf28

    o2 sensor at 1500-2500rpm
    http://www.screencast.com/users/JonS...7-d638fae2db3e

  2. #2
    Join Date
    May 2007
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    5,240

    Default

    The graphs don't look quite right but, It might be the time scale of them. If you look at them the post cat sensor is just mirroring the pre cat sensor so I'd say your cat is in need of replacement.

    Do check first that there is not a misfire or your new cat will soon be ruined.

  3. #3
    Join Date
    May 2008
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    8,390

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    From the graphs it shows both sensors 'switching' at roughly once a second or so. That means the voltage is going from 0.8v to 0.1v. As the second sensor is switching just like the first one, this is proof that the catalyst is dead. the sensors are okay. On this engine, a misfire usually causes premature failure although cats are only lifed to 50,000m. So you need a new cat.

    MrDanno posted at the same time with the same answer!
    Last edited by Hometune; 06-04-11 at 23:16.

  4. #4
    Join Date
    Apr 2011
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    Thanks for the replies MrDanno and Hometune. Yeah I have a feeling you may be right. The car is, however, driving fine and has not had any misfires or stutters. Would you recommend taking it to get the emissions checked, just to confirm it is the cat? Also if the cat needs changing can you recommend where to take it to (i.e should it be done at a Ford dealer or somewhere like kwik fit etc). Thanks very much.

  5. #5
    Join Date
    May 2007
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    5,240

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    A good exhaust centre or an independent garage could do it. My local 'Longlife stainless' centre does very good prices.

  6. #6
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    May 2008
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    Quote Originally Posted by jimmyc View Post
    Would you recommend taking it to get the emissions checked, just to confirm it is the cat?
    Checking the emissions at a test centre will probably not show a fault as the limits are so high compared to Ford's spec. Very often the car will still pass an MoT with a P0420 code showing. But, with this code and a switching second sensor, the cat is definitely past its sell-by date. If you buy a cheap-cheap one from an unknown source, it will last a couple of weeks.

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