most mot failers on cats is down to emissions i have answered a few threads on here and sovled the issue for a few members this a cheap option to try and most of the time it works if your doing stop start journeys it wont do your cat good as it will clog up with soot once a month give it a good hard drive get it hot this will clear the cat .what i reconmend you do is go and buy a bottel of a good branded cat cleaner all you do is add it to your petrol tank put a full bottle in when you got a quarter tank of fuel then give it a good drive refill quarter a tank then put another bottle cat cleaner drive normally take it for your mot should pass on emissions . learn from this once a month put cat cleaner in your fuel i do every month .