Glennage

After failing MOT on high CO ( between 3 and 1.8%) and low Lambda(1.09-1.94) i.e rich mixture, having spent far too much money on new bits that made no difference I am faced with a dilemma:
I am convinced it's MAF (no fault codes), however have tried another known good one and no difference to CO and Lambda, although have to say car felt better even though its running very well with alleged problem one fitted?!
Do I have to do an ECU reset when new one is fitted so that it can learn the limits of the new MAF straight away, or normally does a new one cure a problem like this right away?

Any help greatly appreciated as am about to scream very loudly with frustration!!

Cheers
Glenn