volkswagen golf 55 plate,engine management light?

[~Evil~{Scotsman}]

the car runs with no problems,but had it checked with a big name garage (KF) they said the cat was gone but its defo not....so had it checked at m.o.t and had fault cleared but lights come back on whats causing this.
Did not get (KF) to do the diagnostics or m.o.t