Engine electronics light

I have been having problems with the yellow light with the spanner through it. I have searched online, and it was pointing to the mass air flow sensor, so I bought a used one, I fitted it this morning, and it does seem a lot better, before the engine would cut out when stopping at lights,etc. and upon restart be slow,sluggish getting upto speed, even without the spanner light being on, at times the emission light would also come on, now it drives a lot better, smoother and quicker, but the damn spanner light came back on, and the rev indicator would not move. It is due MOT this month, so any clue would help me please?