Jeff Wisnia wrote:
Over the years I've had to replace maybe a half dozen thermal fuses
which appear to have failed without ever being heated beyond their rated
temperatures.

Most recently a small electric space heater used in our office under my
desk stopped working because its thermal fuse had opened. The fuse was a
MICROTEMP G4A0 121 degree C unit, and I'm pretty sure that nothing had
blocked air passage through the heater and caused its internal
temperature to rise enough to open the thermal fuss.

They seem to have poor aging characteristics. I've seen dead thermal fuses
in completely unused spares heater assemblies sitting on shelves for
years. Everything was x-rayed so new ones could be custom made. Told them
to leave out the thermal fuses on the new ones.