iPhone Overheating

I have the iPhone 5S (Yes, I know it's old, but it was still holding out for me, so I am using it until the day it no longer works.) Anyway, the other day, I went to the store, and I left the phone in the car because it was charging from, like, 4%. I was in the store for maybe half an hour, and when I got back to the car, I turned it on to make sure I hadn't missed any calls, and all I saw was, "Temperature: iPhone needs to cool down before you can use it." The phone itself had only charged to 10%. I unplugged it, because it stopped charging, and powered it off. When I turned it back on after holding it in front of my car's A/C vent, it still said that. It has been about four days now, and no matter what temperature it's in (yes, even the freezer for a few minutes), it is somehow stuck on that screen. Is there maybe an internal thermometer or something that possibly malfunctioned? Can it be fixed, or is it finally time for the 5S to retire?