Tech A says quartz crystals under mechanical stress will produce a voltage. Tech B says the piezoelectric principle is used in coolant temp sensors. Who is correct?

Tech A is right.

Explanation:

Its the ability of quartz crystal that if we apply a mechanical stress on them by an applied force then they can produce the electricity or voltage. This phenomenon of producing electricity by squeezing a crystal is termed as piezoelectricity or a piezoelectric effect.