A method for monitoring the proper functioning of a temperature-dependent resistor, particularly a NTC resistor, for measuring the temperature of a medium heated by a heat source. Said medium is preferably the air flow in a clothes dryer for drying clothes. In order to increase the safety of a device, in particular a clothes dryer, where the temperature-dependent resistor is used in combination with a heat source, it is checked, once the heat source is switched on, whether the temperature measured by the resistor after a first predetermined period of time following the switching on of the heat source falls below a predetermined temperature threshold value. If this is the case, monitoring of a resistor-temperature curve characteristic of the temperature-dependent resistor is carried out.