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On Monday, March 14, 2016 at 12:56:59 PM UTC-4, 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.

Jeff

They most certainly do die of old age. As do regular current-limited
fuses. If ever one wants some very dry amusement, bring a glass fuse close
to its current limit and watch the filament dance inside the glass.

Thermal fuses are often installed with crimp connections to avoid high
failure rates during soldering.

Occasionally, a bad crimp makes a resistive join that heats up and pushes it
past the trip point.