Quote Originally Posted by rumpfy View Post
I think the MAF has a hot wire and a reverse biassed diode. There is not a lot to go wrong with it!!
MAFs in the 2nd & 3rd gens are the Karmann Vortex type. They incorporate some electronics, including a pressure sensor and amplifier, to generate a frequency signal from the Karmann vortices created in the body of the MAF.

A build-up of crud on the airflow straightening honeycomb or the various inner surfaces of the MAF body can affect the vortex generation (and therefore distort generation of the frequency signal) and also block the passages to the pressure sensor resulting in no frequency signal output. The electronics also age (particularly certain types of capacitors) resulting in eventual failure.