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Old 08-04-2010, 12:02 PM   #1
gaz1250
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Ford Focus O2 Sensor/CAT and Code P0420

Hi

Ford Focus 2003 Petrol 1.6 (UK Model)

My car has been running rough for the best part of a year!! Finally it has thrown code P0420.

I have been told that this could relate to either a O2 Sensor or the CAT.

Would I not have another code apart from the P0420 if it was the O2 Sensor as code P0420 is Catalyst System Efficiency Below Threshold (Bank 1) which I would suggest is the CAT at fault?

Also I might be wrong but the car seems to run better now the code has been thrown up which I find very odd?

Any thoughts people?

Many Thanks


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Old 08-04-2010, 12:44 PM   #2
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My best guess is that the downstream sensor, the one fitted after the catalyst , is faulty. This would make the system think the catalyst was not working correctly. Unless you have put leaded fuel in the system, are burning lots of oil, been running very rich for a long time, or have damaged it physically, then the cat is unlikely to be faulty. Replacement is simple.....
It is probably appearing to run better because the system has reverted to 'limp home' mode which gives altered timing and a slightly richer mixture.
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Old 08-04-2010, 12:57 PM   #3
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Ok thanks

The car has had a very bad misfire before the car was running rough or the code appearing.....

If I get the Sensors checked and they are ok it will be the CAT right?
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Old 08-04-2010, 01:08 PM   #4
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The code appears because the downstream sensor is giving a similar output to the upstream, which implies the catalyst is not doing its job. If you can find someone with the expertise to confidently check the sensors and they prove OK, then by implication the catalyst is faulty. But, as I said earlier, the chances are the cat is OK....
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Old 08-05-2010, 12:34 AM   #5
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^^ x2

If it's an original cat and hasn't been subjected to poor long term engine mismanagement conditions ( that 'one year' time frame would worry me).

You can test the O2 primary sensor fairly easily. I'm not completely sure of the secondary - can it be tested the same also?

http://www.ngk.com/glossary.asp?kw=H...aced?&manID=13

I've tried testing two CAT converters on the car with a vacuum gauge.
Compare the in. of Hg. at idle and then 2500 rpm.

Supposed to get a 10 in. Hg. or better increase. Never got it - not even close. Held the rpm constant at 2500 - no increase. Something doesn't sound right.
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Old 08-05-2010, 12:42 AM   #6
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How can you test the efficiency/function of a cat converter- with a vacuum gauge?????
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Old 08-05-2010, 01:16 AM   #7
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You can't - well actually if you consider 'blockage' as having an effect on efficiency/function then you absolutely can.
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Old 08-05-2010, 08:40 AM   #8
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More than likely its your O2 sensor after your cat. On mine it has the same error code however I do not have any performance issues so it could be the cat for you. I have had the code for about 2 months now with no performance problems at all. Bringing it to the dealer Monday hopefully. I've kept putting it off becouse I never had the money for the deductable. I know lame excuse.

Do you get the "rotten egg" smell when you gun it?
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Old 08-05-2010, 11:57 AM   #9
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Hi

No rotten egg smell but bad idle and poor performance are making me think its the cat. I'm going to get live readings of the o2 sensors and report back.
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Old 08-05-2010, 12:40 PM   #10
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Posted via FF Mobile Carts & Horses..... The poor running condition over the course of a year could be the cause of the poor Cat condition which finally "threw a code". In personal experience even a badly plugged Cat didn`t affect engine operation until so plugged there was a loss of power & almost no flow out the tailpipe - No rough running or idle problems at all.....
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