A thermistor is an electrical component that consists of two wires a casing in which the wires are connected to two materials, typically metals, that have different electrical characteristics at varying temperatures. The thermistor, therefore, has a resistance that varies with temperature. Household appliances that need to monitor temperature, such as a dryer, use thermistors to determine the amount of heat to supply and when the heat present matches the temperature setting. By knowing how to check a thermistor, you can often troubleshoot an appliance. And because thermistors are relatively inexpensive, especially when compared for example to a heating element, checking a thermistor can save money.
Remove the thermistor from the circuit in which it is used. For a household appliance, such as a dryer, this can typically be achieved by unscrewing the terminals to which the thermistor wires are connected. Other thermistors can have plugs which can simply be unplugged.
Plug in your ohmmeter probes to your ohmmeter and turn it on. Most ohmmeters have automatic rangefinders, but if you need to select a range, start with the highest range and then adjust the range downward after making your resistance measurement.
Connect the ohmmeter probes to the thermistor. If you get a resistance reading, the thermistor has an extremely high probability that it is not defective. Most thermistors that are defective will be open and show infinite resistance.
Turn on the blow dryer and direct the air flow at the thermistor. The resistance measurement on the ohmmeter display should decrease. A typical thermistor will decrease from about 10,000 ohms to about 1,000 ohms, although these values can vary with the thermistor model.
Disconnect the thermistor from the ohmmeter probes and turn off the ohmmeter.
Use a small plastic bowl when working on household appliances to keep small parts in so you don't lose them.
Household AC voltage is very dangerous and can cause electrical shock.