How to check the serviceability of the thermostat in the refrigerator

A sudden increase in temperature in the refrigerator compartment, the formation of ice on the walls, or continuous operation of the compressor often indicates a failure in the climate control system. Thermoregulator (or thermostat) is a small but critical element that is responsible for turning the motor on and off depending on the set parameters. If this sensor fails, the refrigerator stops responding correctly to changes in the internal environment, which can lead to food spoilage or freezing of the evaporator.

Diagnostics of this part does not always require calling a technician, since there are testing methods available at home. In this article, we will look in detail at how to understand that the problem lies in the thermostat, what tools are needed for accurate diagnosis, and whether it is possible to extend the life of the unit without a complete replacement. It is important to understand that incorrect operation of defrosting systems or a temperature sensor is often disguised as other malfunctions, so the accuracy of determining the cause of the breakdown plays a key role here.

Before you begin disassembling, you should pay attention to indirect signs that are almost 100% likely to indicate a malfunction. mechanical thermostat or electronic sensor. The first and most obvious symptom is the situation when the compressor works without interruption, trying to achieve non-existent cold, or, conversely, remains silent for days, allowing the chamber to warm up to room temperature.

The second sign is a change in the nature of the unit’s operation. If you notice that the refrigerator starts to turn on too often or, on the contrary, too rarely, and at the same time the food in the chamber is either frozen or thawed, this is a sure signal that the settings have gone wrong or the control element has broken down. You should also be wary if, when you turn the control knob, you do not hear a characteristic click, which usually accompanies a change in operating mode.

The third important point is external manifestations. The formation of a “coat” of ice in only one place or, conversely, a complete absence of cold while the engine is running may indicate that capillary tube the thermostat has lost its seal or has moved. In models with electronic control, an error may light up on the display, indicating a problem with the temperature regime, which also requires immediate inspection.

To carry out high-quality diagnostics, you will need to prepare a workplace and a minimum set of tools. First of all, of course, it is necessary to turn off the power to the refrigeration equipment by unplugging the cord from the outlet, since any manipulations with live electrical circuits are strictly prohibited by safety rules.

The second thing you will need is a set of screwdrivers (phillips and flat) to dismantle the panels and remove the thermostat casing. The third and most important tool is multimeter (tester), which will measure the resistance of the circuit and determine whether current passes through the sensor contacts. Without this device, an accurate check is impossible, since a visual inspection often does not provide a complete picture of the condition of the internal contacts.

It would also be a good idea to prepare a container with warm water (about 50 degrees) and a container with ice or simply access to the freezer if you plan to test the sensitivity of the sensor. Have a flashlight on hand, as the inside of refrigerators is often dark, and markings on parts may be small and difficult to read without additional lighting.

The process for gaining access to the thermostat may vary depending on the design of your refrigerator. In most mechanically controlled models, the thermostat is located inside the refrigerator compartment, often hidden behind a plastic panel or decorative trim near the light bulb or temperature control knob.

In some cases, especially in older models or units with a system No Frost, the thermostat may be located outside, on the back wall of the case, next to the compressor. To get to it, you will have to remove the back metal cover. In modern electronically controlled models, the sensors are often mounted directly into the chamber wall or hidden behind air ducts, which requires deeper disassembly.

⚠️ Attention: Before removing any plastic panels inside the chamber, allow them to warm up a little if the refrigerator has just been running. Plastic becomes brittle at low temperatures, and careless movement can lead to cracks or chips that cannot be repaired.

After you have reached the unit, carefully inspect its external condition. There should be no traces of melting, corrosion or mechanical damage on the body. Pay special attention to the entry point of the capillary tube (if there is one) - there should be no traces of oil there, which may indicate depressurization of the freon circuit of the sensor.

The most reliable way to check the serviceability of the part is to test the contacts using a multimeter. First you need to remove the terminals from the thermostat terminals. Usually there are three of them: one general and two working, or two in simple models. Before starting measurements, make sure that the multimeter is switched to resistance measurement mode (Ohms) to the minimum limit, for example, 200 Ohms.

At room temperature, the contacts of the mechanical thermostat should be closed, and the device should show a resistance close to zero (or emit a sound signal in dial mode). If the device shows one or infinity, this means an open circuit and the part must be replaced. However, the absence of a bell at room temperature is not a death sentence, since the contacts can open when heated.

To be completely sure, conduct an experiment with changing the temperature. Immerse the sensing element (bellows or capillary tube) in a container of ice water or place in the freezer for 10-15 minutes. After cooling, the contacts should close (if they were open) or remain closed. Then heat the sensor in warm water - at this moment a click should be heard, and the circuit will open, and the multimeter will show infinite resistance.

If during the test you find that the contacts do not respond to temperature changes or are constantly in one position (closed or open), this is a clear confirmation of the malfunction. In such cases, repair is impossible; a complete repair is required replacement of the thermostat to a new one, similar in parameters and fitting dimensions.

It is important to understand the differences between mechanical and electronic thermostats, since the methods for testing them are somewhat different. Mechanical devices, such as TAM 133 or K-59work on the principle of expansion and contraction of gas or liquid inside a bellows, which physically opens the contacts. Their verification is described above and is quite simple.

Electronic sensors (thermistors) operate on the principle of changing resistance depending on temperature. They do not have clicks and moving contacts in the usual sense. To check them, it is necessary to measure the resistance at different temperatures and compare the readings with the table of ratings for a specific refrigerator model. The resistance should change smoothly when heating or cooling.

Below is a table of approximate resistance values ​​for standard thermistors used in many modern refrigerators. Please note that these data may differ for different manufacturers, such as Indesit, Bosch or LG.

*Status at -10°C depends on the specific setting of the controller response threshold.

If you plan to replace a faulty part, you need to choose the right analogue. The main parameters for selection are the length of the capillary tube (for mechanical models), the number of contacts and their location, as well as the temperature range of operation. Installing an unsuitable thermostat may result in the refrigerator not working correctly: it will either not turn off or will not be able to reach the desired temperature.

When installing a new element, it is important to carefully install the sensitive part. The capillary tube must not be bent at an acute angle, twisted or pulled with force, as this will lead to its rupture and leakage of the working substance. The tip of the tube must fit tightly to the evaporator or be inserted into a special technological hole all the way in order to accurately read the temperature.

Why can’t you use universal thermostats without modification?

Universal thermostats often have a different temperature response range and capillary length. Installing such an element “as is” can lead to the refrigerator turning off at +2°C, without having time to cool the food, or working down to -30°C, turning the chamber into a glacier. Fine adjustment or selection of the original is required.

After installing the new part, reassemble the housing in the reverse order, remembering to secure all wires with clamps so that they do not touch moving parts or hot elements of the compressor. Turn on the refrigerator and monitor its operation for several hours, checking the cycle of switching on and the temperature in the chamber.

There is a common misconception that if the refrigerator has stopped freezing, then it is enough to simply turn the thermostat knob to maximum. This action will not only not solve the problem of sensor failure, but will also create an excessive load on the compressor, which will work for wear, trying to achieve unattainable values. At best, this will lead to the activation of the thermal protection relay, at worst, to the combustion of the motor windings.

Another mistake is an attempt to “reanimate” the old thermostat by tapping or heating with an open fire. Mechanical damage to internal membranes or loss of tightness of the freon circuit cannot be eliminated by such methods. Moreover, heating from an open flame can damage the plastic housing and wire insulation, creating the risk of a short circuit.

⚠️ Attention: Never attempt to repair the sealed bellows of a mechanical thermostat or solder cracks in the capillary tube. These are units designed for a certain pressure, and handicraft repairs do not guarantee safety and durability. The only solution is replacement.

Also, do not ignore the oxidation of contacts. If you see green deposits or rust on the terminals, they must be cleaned before installing a new sensor, otherwise the resistance in the circuit will increase, which will lead to incorrect operation of the entire control system.

Is it possible to temporarily operate a refrigerator with a faulty thermostat?

Theoretically, it is possible if you manually turn the refrigerator on and off from the outlet, focusing on the temperature products or an external thermometer. However, this is extremely inconvenient and risky: it’s easy to miss the moment and defrost food or, conversely, overload the compressor. In addition, in models with No Frost, defrosting will not turn on, which will quickly lead to ice fouling on the evaporator and damage to the fan.

Why does the new thermostat click, but the refrigerator does not turn on?

If you installed a new part, you hear clicks, but the compressor is silent, the problem may not be in the thermostat. Check the compressor start relay, the integrity of the motor windings or wiring. It is also possible that the new thermostat does not fit the parameters (for example, it has normally open contacts instead of normally closed ones at room temperature).

How often should the thermostat be changed for preventive purposes?

There is no special prevention schedule for thermostats. This unit is replaced only when it fails. The average service life of a high-quality mechanical thermostat is 7–10 years, electronic sensors - up to 15 years or more. Regular defrosting and cleaning of the condenser helps extend the life of the entire system, but does not prevent natural wear of parts.

To summarize, we can say that checking the thermostat is a task that is feasible for a home craftsman with a minimal set of tools. The main thing is to act consistently, not to ignore electrical safety rules and to correctly interpret the multimeter readings. Timely diagnostics allows you to avoid expensive compressor repairs and loss of products.