I had been having an issue with my coolant-level sensor triggering when going
over bumps: The idiot-light would come on, I would pull over, and the coolant
level would be fine. Giving the expansion-tank a smart rap would turn the
light off.

I investigated more closely, and found that the wires to the sensor are "potted"
with some sort of rubbery substance, and that there was some play there. I
pushed the wires in to their maximum extent, and all has been well since.

Apparently, the potting can get soft with heat, and if the wires are tugged in
this state, the height of the magnetic sensor becomes too low, and then the
magnetic float doesn't quite have enough strength to close the switch (the sensor
is very low resistance in the presence of a magnet near the tip).

Just thought I'd pass this on to those who are having this relatively common
problem: Not necessarily any need to purchase a new tank or sensor.