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P0420 Catalyst System Efficiency

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I have a 52 plate Petrol Ford Focus and have the following error code:

P0420 Catalyst System Efficiency I understand this relates to either the O2 Sensors or the Catalytic converter. The car has 80,000 miles and has not been serviced for sometime. It only does short journeys. 10 miles a day.

What I would like to know is:-

1 How do I know if the Catalytic Converter is damaged or if it is one of the O2 sensors and how do I determine which sensor. The live data show that the sensors run at similar rates.

2. The sensor in Bank 1 is that the same fit as Bank 2 or do I have to buy a specific one for Bank 1 and 2

3. I have cleared the error the car is running fine should I see how it goes and will the light come back on if is the sensor or catalytic converter?

4. I have seen some cheap sensors on ebay but have read some horror stories about not fitting genuine parts what are your thoughts.

I need some expert advice.


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Hello and welcome to the forum,

You say the sensors run at a similar rate, can you give us any more info on that?

The front sensor should be switching fairly quickly between 0.2v and 0.8v. The rear sensor should display more of a steady voltage

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I can post some live data tomorrow for you if that would help.

Do you know if it is ok to drive with this error, saying that I cleared it yesterday and it has not reappeared.

Thank you for your help


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cat efficiency will refer to the cat or the after cat sensor, not the pre cat sensor

the code can trigger due to a brief problem, for eg, a minor and brief glitch in wiring on secondary sensor, once that code is logged, it wont clear itself even if the fault is no longer existing.

so wouldnt be too concerned if fault hasnt reappeared since clearing, just see what happens.

there is no bank 2 on a focus, as its an inline engine, bank 2 will be for v type engines, the pre cat sensors will be the same be it bank 1 or 2, as will the post cat sensors...just bear in mind that pre cat and post cat sensors are normally different.

if you do end up needing a sensor, then yes, stick to known brands such as NGK/Bosch as some of the cheaper makes are not worth the saving

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