O2 sensor high emission and error question

Hi all my 2001 F (abbingdon 1.8 mpi mk2) is every year hard to get through MOT emission tests.
Testing it with warm engine and adding Injection Cleaner takes is just below CO threshold.

To that I get regularely this warning: P0420 Catalyst System Efficiency Below threshold.


Usual suspect is the o2 sensor... attached some graphs of Torque where it looks like o2 sensors do seem to work....
If not good which one will needs to be replaced? Or has my CAT maybe died?



Please help me debugging and advise me on where to start fixing this.
Thanks in advance!

Does this suggest the Cat may need replacing ???

Yes that is the question. Or is there anything else to replace, adjust ...?

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The fluctuations should be greater on the first sensor. Both sensors should go between 0.1v and 0.9v.

Is it an oem cat or a cheap replacement? These symptoms suggest low cat efficiency due to insufficient reactive material.
I agree that the pre cat sensor readings are also suspect.

From a costs perspective I then better first replace the precat sensor, measure the values and get my MOT emmission test. And if emissions are on the edge again I will order a new cat.
As far as I know I have the original cat under the car but I haven't checked visually. The car has run 120.000 kilometres so that's not a huge amount, normally the cat should last this long but maybe I have bad luck on that.

Check question, just to be sure: O2 sensor 1 is the precat sensor, right ?

Thanks for your help!

Yep.:yesnod: