Our 03 Freestar minivan didn't use pressure sensors, it used a variation on
the ABS idea...it used the ABS signals from each wheel to determine an
average wheel speed differential while driving, and if any tire ran on
average faster than the others, that would indicate a loss in diameter, ergo
a loss in air pressure.

We drove to D.C. when the damn thing lit up,  after the fifth time I checked
the pressure on the Ohio Turnpike I decided it was a false alarm, since the
pressures were alway spot-on. But  I couldn't figure out how to reset the
thing, you have to turn the key a few times, press the dash disply button a
few times and say fifteen Hail Marys, or something....