Ive found with cars that burn oil that the oil being burnt tends to kill the cat. We had a pug 106 in that had not been serviced in years and had started burning oil because of this. It failed its mot due to the cat not working but there was no signs of smoke being immited from the exaust so we changed the cat and gave it an oil and filter change (using slitly thicker oil) and it passed no problems probaly need another cat next year tho lolol