I bought the car in February, the car dealer (not renault) said he had just had to put a new CAT on the car, the car ran perfectly well until Renault got their hands on it.
i have not looked under the car and have no intentions of, the problem only arose when Renault fitted their computer to my car to "reset the fault light"
i personally wouldnt know what a cat looks like anyway, my experience on fixing cars was pre- cats and fancy computerised engine management. when all this started coming in, i hung up my tools, it all seems to be aimed at making money for the dealerships if you ask me
the Dealer didn't change the CAT, but did advice on other items that were in need of replacement such as 4 new tyres, a full air-con overhaul and a new timing belt, a total of over £1200 if they had done it and that was without a new cat.
all the troubles started when Renault had my car in their garage, so i am assuming it is the computer being connected that has caused the problems.