It's just a simple bi-metalic switch, so it's not the highest failure
rate widget.   It opens up when the heater air gets warm and turns off
the little aux heat pump.   I can look up the exact temp for you, but
as I recall it should go from closed to open around 70-80 degrees.
You can check it with an meter and if it opens/closes around there, its
good

Don't know for sure, but I would think they are fairly reliable.  It's
just a simple bi-metalic switch that's widely used in all kinds of
apps.  Plus it's located in a secure, dry location inside the car.
Mine is 26 years old and still works.

On the other hand, the aux water pumps are real pieces of crap and have
a pretty high failure rate.  With the car turned off, you can use a
couple of jumpers to power the aux pump directly from the battery and
you should hear it run.  If it doesn't then the pump is kaput.

Start the car up when it's been out at night or day when you know it's
50 or below.   There is an easy to pull connector on the wire going to
the aux pump.  Verify that you have 12V coming to the connector.
Reconnect it and drive the car for a couple miles with the heat on.
When you return, pull the connector again and you should have 0 volts.
 You may also be able to tell if the pump is not running from sound,
but it can be hard with the engine running.

 
If it passes tho