I bought a car last March with a fresh MOT. The car had failed the MOT on emissions and the previous owner paid the garage to fit a new cat and retest which obviously it passed. I have the invoice for the work carried out.

I’ve just put the car in for this years test and again it has failed the emissions on high CO levels meaning the cat is not working properly. The car has no running issues etc. That could have damaged the cat.

Leaving aside the issue of it being the previous owner that got the garage to fit the new cat, where do I stand with regards to going back to them? Surely a cat is expected to last longer than a year, even the cheapo ones.

and how are you so sure the CAT is the problem? Could be other things

CO levels can be more than the cat, I think.
How much did it fail by? My old Fiesta, M reg 1.4 8v was always close to the emissions limit and would occasionally fail. Once or twice I think it then passed with just an ‘Italian tune-up’.
I think it also failed once ‘cos of a dodgy lambda sensor which I think gave a high CO reading…

Faulty lambda sensor can damage the cat too, I think.

Reckon you’ll struggle to get anything out of the garage that fitted it.

Lambda was replaced at the same time as the cat.

Fast idle test showing 0.77% against max of 0.3%.

HC passed
Lambda passed

The test station recommended replacement of the cat even though they said they could see it was pretty new.

The trouble is with aftermarket cats they are pretty crap quality. You’ll probably find you have a new looking shell with nothing in the middle!

The Walkers cats are so light compared to the originals its shocking!

As above , after market cats 99% don’t pass !!! It’s a joke that the faulty cat is worth £50 and I can buy a new ” after market ” unit at trade for £60+ vat .
I’ve sent so many customers back to quick fit for a second and third cat due to failures and in the end quick fit does the mot for free and suprise suprise it passes !!!
Cheap non gen cats never pass unless the garage that fits it ” makes ” it pass obviously its in there interest to let it idle through the emissions test rather than 3000 rpm .