The thermistor is like a resistor that changes resisting value based on temperature. If a thermistor fails, the voltage returned will be wrong - so the circuit looking at the output will think the temperature is something else (and perhaps out of range).
Thermistors generally cost under a buck. Figuring out that the thermistor needs replacing and then replacing it could cost you a hundred times that.
Tony you are the font of all knowledge!! Bloody amazing.