Thermistors for Fridge and Freezers
Thermistors are used to control the temperatures in both modern fridges and freezers These have replaced the old analogue thermostats that were previously used to control the temperature, but allow far greater control as they can read a range of temperatures rather than specific preset temperatures found on the old thermostats.On the more technical side, there are two types of thermistors. One that is called a positive temperature coefficient (PTC) and the other is a negative temperature coefficient (NTC). The most commonly used thermistor is the NTC, and is found in many appliances.
Reliability and Testing
Apart from a few exceptions, thermistors are very reliable. However, if you are experiencing problems with either your fridge or freezer, it is fairly simple to check if you are technically minded and familiar with using a multimeter.
If, however, this is not something that interests you, your local engineer can certainly help you to identify what the problem could be.
Either you, or your engineer, will connect the multimeter to show a resistance value. You can then change the temperature of the thermistor by cupping it in your hand, or placing it near a cool or hot source, and watch the multimeter for any change in resistance.
If you see a change in the resistance, it should indicate that the thermistor is working properly but if there is no change in temperature, it generally means the thermistor is faulty.
Having isolated the thermistor from the temperature problem, it is more than likely that the electronic control card has failed, but there are still a few more things to check first.
You need to make sure the connections are all clean and correct on the board. Also ensure there is nothing touching the thermistor that could cause a false reading.
Regardless of whether your appliance is frost free, defrost the fridge freezer thoroughly as a certain amount of ice can build up and cause the sensor to give incorrect values.
If all the above checks out, it is more than likely the board is faulty. Both thermistors and control boards are readily available as spares but you may feel that this is a job for a professional.