Why do crank pos sensor fail?

Hmm.. Seems a bit odd. I have changed the crank sensor once but has had no other issues with it. I am absolutely no professional, but the only thing that strikes me is if the voltage to the sensor might be high/low? But that also seems a bit odd. You could try to measure the voltage while revving the engine and see if it changes. You can put wires between the harness and the sensor connections and try (with a helping hand) to measure between (I think the pin closest to the flat side of the connector) and try both of the other pins to see which one gets a voltage.

This might be quite difficult, but it is the only cause i can think of.

Good luck,

sure its sensor and not the wiring to it?

age may be , mine was on its last gasp . the local engine shop , found the out put voltage dropped at high rpm , new one and lots of odd things disappeared , later swapped the cam one as well for new one , not had the mythical poo12 cam ****** flag since ....... the cosworth v24 had similar problems with the crank sensor at this age