hi a friend of mine has an octavia vrs (black 51 plate) and in the last few weeks his management light has started coming on with code p0420 reading" cat efficency below thermal bank 1" in memory. this doesnt mean a lot to me or him. only other thing is he did his cam belt a few weeks ago so he wondered if his timing was slightly out could it cause this?,:confused: as the time of light coming on coincides with belt change. if not any ideas???? he aint on line hence me sending post. help woud be v.appreciated thanks

Normally an O2 sensor failure I think -

16804/P0420/001056 - Ross-Tech Wiki

Timing out on an AUQ usually brings up a cam/crank correlation error. I should think that the cat efficiency is calculated from data obtained by the oxygen sensors, this would be a starting point - also check for any exhaust leaks. Can you get it on a gas analyser and see what the lambda is up to?

yes im an mot tester so ill get him to bring it into work and check emissions thanks