All appliances that need temperature regulation need some way of sensing the temperature inside the cavity that is being used.
In the case of fridges 2 main types are used, electro mechanic bellows type thermostats and thermistor digital NTC sensors.
The bellows electro-mechanical type
Have been used the most over the years as they are cheap and reliable but they are not the most accurate and the control is basically an on and off switch. It operates by the principle of expansion of gases. As the temperature around the phial changes the gas contracts and expands, this movement is use to turn the switch on or off . There is a dial usually numbered 1 to 5 this can be used to alter the distance between the contacts and therefor it will to a certain level of accuracy give less or more cooling time to control the temperatures in the cooled cavity.
The phial of the thermostat must be located on the evaporator to sense the temp this is why buying the exact OEM replacement is wise as some universal ones can be too short and as such would not be in the correct position to read temperature and this would affect cooling seriously. ( remember if its a few degrees out then food could become poisoned )
You can do a basic test on these try our post here for more information on this please.
What can go wrong with these, they are quite reliable but as mentioned before they can be too small if you buy cheap replacement, they could be designed internally for differing temperature ranges , the phial/capillary itself could develop a leak and lose its gas which would mean over freezing as compressor would not turn off.
It could also fail in the open position and not run at all see above post for multimeter tests, if you know what your doing and only if you know you could jump the wires to see if compressor switches on , you need to connect the live to the switched live only for a very short time but beware you could cause damage or injury so only attempt if you know what you are doing.
Digital thermistor temperature sensor ( NTC )
Most modern and top of the range fridge freezer units now use a digital method of sensing the temperatures these use the thermistor NTC type sensor.
The sensors monitor the fridge freezers cavity and feedback to the control board the temperatures, there can also be other sensors for defrost and outside temperatures and sensors for different compartments. This is so the control board can have all the information needed to control the refrigeration cycle accurately.
NTC negative temperature coefficient as temp rises the resistance falls, this can be used in the testing process.
To test you need the multimeter as the sensors use resistance you will need to set the meter to read ohms. Take a measurement at room temperature and compare this again manufacturer specifications you can then cup your hand around sensor to warm it up at it should if working correctly drop in resistance.
What can go wrong with these sensors, They can go open circuit internally or in the wiring, this can be confirmed by using the multimeter set to resistance and take an ohm reading it could also create an error code to flash up on the fridge freezers display panel.
Sometime the insulation can break down around the sensor and water can get inside this will make the sensor read erroneous measurements and will then affect the cooling of fridge.
Replacement is the only option with these and they are no expensive anyway but always buy OEM
Video below shows process of testing both thermostats and digital temperature sensors thermistors
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