Question

the purpose of the outdoor air temperature sensor on a heat pump is to tell the heat pump when to come on to shut down the heat pump when the outdoor temperature is too low to tell the defroster when to come on to activate the reversing valve to prevent freeze - up of the outdoor coil

Explanation:

The outdoor - air temperature sensor on a heat pump monitors the outside temperature. When the temperature drops to a certain level, it signals the defroster to turn on to prevent ice build - up on the outdoor coil. It is not mainly for telling the heat pump when to come on (that's more related to thermostat settings), shutting down the heat pump at low temperatures, activating the reversing valve, or just preventing freeze - up in general (the defroster activation is a key part of preventing freeze - up).

Answer:

to tell the defroster when to come on