Paul,

Put a multimeter on the sensor wires on resistance.
Observe the value the wiggle the wiring vigourously and see if the values change significantly.
Often the sensors fail not because of the sensing head but the wire itself because it gets a pounding from all the
bumps in the road and when the steering is turned.

I had this twice on my sierra and managed to fix them by replcing the cable by cutting of an inch at a time and retesting.
(Carefull of polarity).

Obviously, if you do this, its at your own risk