My new GT suffers a similar but intermittent problem, sometimes it reads -39.5oC for a few days then its back to normal again. Its going back to the dealer when it stays broken, its OK now.
Be advised that it may not be the sensor at all, as in my case I have checked this both when its working and in the bad state and it reads the same resistance.
Try to measure the resistance across the two salmon coloured wires in the wing mirror connector with the connector disconnected from the car loom. Depending on temperature it should be somewhere between 100K and 200k ohms.
Assuming you can buy a sensor ,I don't see why one cannot replace the sensor, as it's fairly easy to remove.