P0420 Catalyst issue

[Daveof49]

Hi. I have the P0420 catalyst issue on my 1.2 Puretech 308.
My mechanic changed the upper 02 sensor (S1) and erased the code only for it to return after about 50miles
I have connected my cheap OBD reader and S1 sensor shows 0v whilst S2 is showing a varying voltage which I think are within the expected parameters for S2. I know S1 should have a varying voltage.
The car runs well. It accelerates well, pulls well up-hills etc however it sounds a little lumpy on tick over and I think fuel consumption is not as good as it was before the P0420 error.
I would say that the error started after driving through teeming rain and puddles and I initially thought it was shock to the cat but as the cat is quite high up in the engine bay I'm not so sure now.
I have also measured the temperature at the cat in and out - around the S1 & S2 o2 sensor locations and the in was at around 280 deg F, the out was slightly lower when it should be around 100 deg F higher. Could this be caused if S1 is faulty or does it mean the cat is doomed?

Is there anyone on the forum able to shed some light on this for me. Any help would be greatly appreciated.

 

[Shaggy]

S1 sensor shows 0v whilst S2 is showing a varying voltage

That 0v on the front sensor is the big clue. A good upstream sensor should be moving around, so unless your reader just can't read that type of sensor, I'd be looking at the wiring and plug to S1 before the cat, especially since it all started after driving through deep water. Water in that connector or a chafed wire would do exactly this even with a brand new sensor. Did your mechanic check for power at the sensor plug?