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P0420 error code help needed

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Hi all

Had this problem with my 2003 1.4 Fiesta for some time but now getting round to try and fix it 

for the last 40k miles every 500 miles or so the EML pops on with an error code P0420 Catalyst System Efficiency Below Threshold (Bank 1)

So how do I find what is the route to the problem, I assume either one or both o2 sensors or the cat itself could be at fault, all quite pricy on a car this age so don't want to go down the try and see route.

How do I test the sensors to double check they are at fault.

I have monitored the sensor voltages whilst driving assuming that this would indicate if one or both is not working or if both have the same reading I understand that would indicate the CAT has failed.

I used Torque to log the data but can only get it to send screen captures not the whole logg 

Looking at the three attached captures do they indicate anything obvious

Thanks for any help resolving this





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[Edit: Apologies if this response is teaching Granny to suck eggs; I have just re-read your post and see that you were pretty much one step ahead in your diagnosis - I think I was so focussed on the plots and constructing my reply that I overlooked the detail of what you'd already surmised!]

The issue appears to be with the cat itself (as the code interpretation is suggesting).

Sensor 1 is pre-cat and thus you would expect it to oscillate between 0.2-0.8v as the ECU shifts the engine between rich and lean running states in closed-loop mode. The tipping point is a very fine line hence the constant oscillation between the two states several times a second.

Sensor 2 is post-cat and so with the catalytic converter doing its job it should see only a gradually-varying change given the pollutants being extracted (converted). What it shouldn't do is oscillate in time with Sensor 1 because this means the cat isn't doing its job. If you imagine the cat not even being there then only then would you expect both sensors readings to be the same.

Your Sensor 1 and 2 plots are clearly not identical and so this suggests there is *some* function still present but evidently not enough. To be honest I am not sure how cat health is checked - perhaps garages just use the very same machines that they use in the MOT to test emissions as that is afterall the sole function of the cat. Perhaps a trip to a garage might therefore be worthwhile?

I don't think (but could be wrong) that the sensors are at fault as I would imagine you would usually see no, or slow, response from them or seeing values outside the expected range if they had issues. The fact that they are responding in an as-expected way, and that they are broadly behaving the same, would to me suggest that they are working as designed and positively indicating that the cat is dead/dying.

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Thanks for the replies, most helpful

Its booked in for its MOT next week, will see if or hor how bad it does then.

If it is the Cat then it might be time to say goodbye unless I can find one at a good price.


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There are quite a few on eBay for around the £50 mark so don't be too put off. There are plenty of cars out there running original cats that are much older than yours so there's no reason to think they too would be shot.

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