Originally Posted by gtsuper

Being a 2017 Mustang GT owner, I have also run across this problem. Since the HVAC system is contingent on the ambient air temperature sensor working properly the air conditioner will not work if it thinks it is 50 degrees when the outside air temperature is actually 90. There is a temporary fix and a permanent one. The temporary one has to be reset every time the car is started. Because of that I will go right to the permanent fix. Note: the dealers don't know about this and this is assuming there is not a hardware issue; you need to re-calibrate the sensor. This will do exactly that.

First, have the engine running. Second, press and hold the power button ie: the off/on button for the radio simultaneously with the sync fast forward button ie: double arrow button. Third, hold for at least 5 seconds. The screen will go blank for a few seconds then come back on. Fourth, drive the car and attain a speed of at least 20 mph. This should re-calibrate the ambient air temperature sensor. I've done this approximately three weeks ago and my sensor is still working fine. Please let me know how this works after you've tried it. GTSUPER.