Ambient temperature reset

My local mechanic replaced a bad heater core in my car, which included disconnecting and then reconnecting the ambient temperature sensor. Now, instead of a temperature reading, I either see three dashes (---) or an erroneous temperature. Friday night it showed a minus 22 (-22). When it shows dashes, my AC works fine. When it showed -22, all I could get was very hot air. My mechanic says the temperature probe needs to be reinitialized, but that only a Subaru dealer can do it. Is that right? Is there any way around paying a dealer's service department big bucks to stick in a probe and run a software fix on my car?