Hiya

Had one of these recently coming up with cat efficiency, replaced the cat which was quite expensive and no problems. Was the cat you fitted type approved? Anything after 2000 has to be type approved, if its not then it will still pass an MOT but will fail the ECU tests after a few weeks or less.

Had a peugeot like that as well, it was on the change year, i couldnt get a type approved cat for it that would fit correctly, it would only do low mileage and would come back every few months, main dealer was the only place to get the right cat but it was too expensive for the customer and the car, it goes through an mot fine so the customer is happy to leave it.