Imbedded under the sealed cover on the other side of the maf from the diode is a circuit board and in the slot at the bottom is a platinum coated wire.
Its the cooling effect on the wire caused by the air flowing over it that determines the readings from the maf which are used by the ECU to determine the correct fuelling.
It's the effect of pollutants and oxidization that burn onto the wire that causes it to go out spec.
Russ