P0420 - Bad Cat or Bad Sensor? MK7

[PricklyBuzzz]

When I first drove my car home (literally the first motorway trip) i get the Engine light on caused by P0420 (cat Efficiency poor) I cleared the fault code and the light never came back. 6 Months and 7000 ish miles later it came back when i was on a long motorway run again (i have done loads of long trips in between with no issue).

The car has covered 58,000 miles and I understand its not uncommon to have to replace the Oxygen sensors which I'm happy to do however i am worried the cat is the issue? Id like to think it wont be seeing as the car has never been mapped/tuned and was always driven with care prior to my ownership. It has never has a misfire which would cause the cat to burn out and maintenance has been carried out under mileage requirements.

So far i have cleared the DTC and cover another 150+ miles with no issues. I've been trying to get an answer on what to look for on a scan/diag tool. Is the downstream sensor supposed to fluctuate or be static?


P.S i know im a newbie, i did try do some searching before i posted but I apologise id this has been covered before and I've missed it!

 

[Sam4]

Swap it out - it's really easy to get to, and you don't need an O2 socket. You'll gain a little together time with your ST swapping stuff you never imagined!

 

[PricklyBuzzz]

Swap it out - it's really easy to get to, and you don't need an O2 socket. You'll gain a little together time with your ST swapping stuff you never imagined!

Haha yea together time usually ends in arguments! I actually have both Sensors so may just change them both as i know they lose efficiency over time. I got genuine ones to, in my experience cheap ones are a false economy

 

[Snrub]

I've had multiple p0420s, in multiple other cars. It was always the cat...

 

[XR650R]

Replace the oxygen sensors. If it still fails, it's the cat.
That ain't cheap, and it shouldn't fail so soon.