PastyKing

i know its not ford related but ill give it a shot.
i recently had my nissan diagnosed by a main dealer and it returned a fault saying fuel system lean, mass air flow meter faulty.
I had the emission control light on.
Nissan reset the light and i bought a new MAF meter for £50 and fitted it myself.
car runs brilliantly now and more responsive but......
the car smells of suplhur quite a bit?
but i replaced the cat last year and im wondering if maybe when the air flow sensor was broken its put extra unburnt fuel through the cat and it needs to have a good drive to burn it off?
any help welcomed as i have a MOT next week