My partner has a heater for heating wax that is used for waxing legs and other body parts. She told me it stopped working; it does not heat up anymore.
When I opened the device and looked at it I found that the temperature is controlled by an NTC. When I measured its value, my meter said that it is open-circuit.
However, when I heat the NTC with a hot air gun, its resistance drops to a few kOhm.
When I let it cool down, its resistance increases and when it reaches 32 kOhm, it suddenly goes open-circuit.
I've repeated the heating and cooling cycle a few times to make sure that it isn't a fluke (although I am using a Fluke multimeter).
I've never seen an NTC behave like this; the resistance normally increases until it reaches its nominal value at room temperature.
Is this a special NTC or is it just broken?
I haven't been able to look at the NTC itself, because it is built into the device and I don't want to take it further apart unless I am pretty confident that the NTC is broken and needs replacing.
The NTC is soldered to two leads and is inside heat-shrink. The assembly is wrapped in transparent light-brown-yellowish adhesive tape. I assume that this is heat-resistant adhesive tape.
Best Answer
This is a quite common behavior of NTCs, i.e. Negative Temperature Coefficient thermistors: probably the device you've at hand is designed to work at high temperatures, therefore its resistance is very high at ambient temperatures, so high that for a normal tester it resembles an open circuit (for example \$\ge 10\mathrm{M}\Omega\$). Here it is a typical resistance vs. temperature behavior diagram taken from the Wikipedia commons:
The vertical asymptote can start even at \$25°\mathrm{C}\$, making the increase of resistance at low temperature very steep.