DuoTherm Heat Pump Icing Up

I have found a service manual online for an older model, but the main components seem similar. There is an ambient sensor that is supposed to send the heat pump into defrost mode once outside temps go below 42. It has a red plug and goes right into the control box. My preliminary testing shows its ohm reading to be within the service manual's specs, around 13480 ohms and it was around 48 outside. But I'm going to check it again tomorrow when I have two extra hands to hold my meter. I'll let you know.