If your car is an early one, then there is a modification to the sensing pipes that stop condensation. This is as far as I'm aware be main reson for limp mode condition and therfore Differential pressure sensor failure.
My 2.0D had two over 110k miles.
The sensor will probably have failed and can be bought from any Ford dealer. Just check the part number stamped on the moulding.
Can't help on voltage output, but there are three wires I think. One will be supply, one return (grnd) and the other signal.
Perhaps you can put a meter on each pin wrt chassis and see what you get.
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