The basis for the stable operation of any modern refrigerator is an automatic temperature maintenance system. The heart of this system is thermal sensor, which continuously reads indicators of the internal environment and transmits them to the control unit. Without this small but critical element, the refrigerator would turn into a simple metal box, unable to keep food fresh.
The principle of operation is based on a physical change in the properties of the sensitive element under the influence of heat or cold. Depending on the model and year of manufacture of your unit, be it classic Indesit or advanced LG with No Frost system, the design of the sensor can vary significantly. Understanding exactly how this process occurs will help you quickly diagnose faults and avoid costly repairs.
In this article we will take a detailed look at the internal structure of various types of thermostats, analyze their physical principles of operation and learn how to identify signs of failure. You will learn why the chamber may be too cold or, conversely, warm, and how to check the health of the sensor with a multimeter.
Physical principles of operation of a temperature sensor
The fundamental basis for the operation of most household temperature sensors is the dependence of the electrical resistance of the material on its temperature. Such elements are called thermistors or thermistors. When the thermal load inside the chamber changes, the circuit resistance also changes, which is recorded by the electronic control module.
There are two main types of thermistors used in climate control technology. The first type is NTC thermistors, whose resistance drops when heated. The second type is PTC thermistors, whose resistance increases with increasing temperature. In refrigerators, they are most often used NTC sensors, as they provide more accurate readings in the range of negative and low positive temperatures.
⚠️ Attention: Never try to check the serviceability of the sensor by pinching it with your fingers or heating it with a hairdryer without disconnecting it from the control board. A sharp surge in voltage or a static discharge can damage not only the sensor, but also the main processor of the refrigerator.
The electronic control unit constantly scans the status of all connected sensors. It compares current readings with user-specified parameters. If the actual temperature differs from the target temperature, the controller makes a decision: turn on the compressor for cooling or start defrosting the evaporator.
Design features of mechanical and electronic regulators
In old ones Refrigerator models, as well as in some budget series, still use mechanical thermostats. Their work is based not on electronics, but on the properties of gases and liquids. Inside the capillary tube of such a regulator there is freon or an inert gas, which expands or contracts when the temperature changes.
This change in volume is transmitted to a bellows membrane, which mechanically opens or closes the electrical contacts. When the chamber becomes warmer, the gas expands, the membrane bends and closes the circuit, starting the compressor. When the desired cold is achieved, the process is reversed. Such devices are reliable, but have a wide range of readings and are subject to wear of mechanical parts.
Electronic sensors, unlike their mechanical counterparts, do not have moving parts. They are a compact element, often enclosed in a plastic or metal case. Modern systems No Frost can use up to ten different sensors that control different zones:
- 🌡️ Air temperature sensor in the refrigerator compartment
- ❄️ Evaporator sensor for controlling defrost cycles
- 💨 Temperature sensor in the freezer compartment
- 🚪 Freshness zone sensor (zero chamber)
The accuracy of electronic systems is an order of magnitude higher, which allows manufacturers to guarantee compliance with food storage conditions. However, the complexity of diagnostics is also higher for them, since they require special equipment to read error codes.
Why do mechanical sensors click?
The click that you hear in old refrigerators is the sound of the contacts opening or closing inside the thermostat. In electronic models, this process occurs silently at the chip level.
The location of temperature sensors in various cooling systems
The location of the temperature sensor directly depends on the type of cooling system and the design of a particular refrigerator. In models with a drip system (crying evaporator), the sensor is usually mounted directly on the back wall of the refrigerator compartment or built into it. This allows you to control the surface temperature of the evaporator, preventing it from completely freezing.
In systems No Frost the architecture of the sensors is much more complex. Here it is critically important to know exactly the temperature of the evaporator in order to turn on the defrost heating element in time. If the evaporator sensor is “lying” and shows a higher temperature than it actually is, the refrigerator will not turn on the defrost, which will lead to ice fouling on the evaporator and stopping air circulation.
Sensors are often hidden under decorative panels. For example, in refrigerators Samsung or Bosch access to them may be closed with plastic plugs or located behind the back wall of the freezer. Replacing them often requires complete or partial defrosting of the unit during the day.
| Sensor type | Installation location | Function | Consequences of failure |
|---|---|---|---|
| Air sensor | Upper part of the chamber | Control of overall temperature | Undercooling or freezing of products |
| Evaporator sensor | On the evaporator pipes | Management defrost | Ice build-up, cessation of cooling capacity |
| Defrost sensor | In the heater area | Turning off the heating element when heating | Overheating of plastic parts, fire hazard |
| Condenser sensor | In the compressor area | Overheat protection | Frequent compressor stops or combustion |
Knowledge of the exact location of the element is necessary for competent diagnostics. An error in determining the required sensor can lead to the replacement of a working part and the persistence of the fault.
Typical signs of a thermal sensor malfunction
Diagnostics begins with observing the behavior of the refrigerator. If the temperature sensor fails, it stops transmitting correct data, and the control unit begins to operate in emergency mode or chaotically. The first warning sign is often a change in temperature without changing the settings by the user.
One of the most common symptoms is continuous operation of the compressor without shutdowns. This occurs if the sensor is frozen in a high temperature state and constantly requires cooling. As a result, the chamber may be too cold, the food freezes, and a thick ice crust forms on the back wall.
The opposite situation occurs when the sensor circuit is broken. The controller receives an error signal or thinks that the chamber is already cold enough and does not turn on the motor. In this case, the food begins to deteriorate, the chamber is warm, but the compressor is silent or turns on for very short periods of time.
- 🔊 The compressor hums constantly and does not turn off
- 🧊 The appearance of ice or puddles in the refrigerator compartment
- 📉 The food in the freezer is melting, although the indicator is on
- 💡 Temperature indicator flashing on the display
⚠️ Attention: If you notice that the refrigerator has started to work in increased mode, immediately check the door seals. Sometimes the problem is not in the sensor, but in the fact that warm air enters the chamber from the outside, and the sensor honestly tries to compensate for heat loss.
A malfunction may also be indicated by a characteristic burning smell or melted plastic in the area where the defrost sensor is installed. This indicates that the sensor did not turn off the heating element in time, which is a critical situation.
Diagnostic and testing methods with a multimeter
To accurately check the health of the temperature sensor, you will need a digital multimeter set to resistance measurement mode (Ohm). Before starting any disassembly and diagnostic work be sure to disconnect the refrigerator from the power supply. Working with electrical appliances under voltage is life-threatening.
The first step is to visually inspect the sensor and the wires suitable to it. Look for signs of oxidation, insulation damage, melting, or mechanical breaks. If there are no visual defects, we move on to electrical measurements. Disconnect the sensor connector from the control board.
Connect the multimeter probes to the sensor contacts. Record readings at room temperature. Then place the sensor element in a container of cold water with ice (temperature about 0°C) or, conversely, heat it in your hands. The resistance should change smoothly.
Approximate resistance values (depending on the model):At +20°C: ~ 5-7 kOhm
At 0°C: ~ 10-15 kOhm
At -18°C: ~ 20-30 kOhm
If the multimeter shows an open (infinity) or short circuit (zero), or if the readings do not change when the temperature changes, the sensor is faulty and requires replacement.
☑️ Algorithm for checking the sensor
Replacement and calibration of the temperature sensor
The process of replacing the sensor requires accuracy and adherence to the sequence of actions. After purchasing a new part, make sure that its markings and nominal resistance match the original. Using an unsuitable analogue can lead to incorrect operation of the entire system.
To replace the No Frost system, it is often necessary to remove the back panel of the freezer. Be careful with plastic latches - they become brittle in the cold. After installing the new element, it is necessary to securely fix it in the seat, often using aluminum tape or a special clamp for better thermal contact.
Some modern models of refrigerators, for example, certain series LG or Whirlpool, require software calibration or error reset after replacing the sensor. Without this procedure, the refrigerator may continue to operate in emergency mode, ignoring the new working element.
⚠️ Attention: Specifications and error reset procedures may vary depending on the specific modification and year of manufacture. Always check the official service documentation for your model before starting work.
After assembling all components, turn on the refrigerator and allow it to reach temperature for several hours. Only after this can you load products. Monitor the operation of the unit on the first day to make sure that the on and off cycles are correct.
Is it possible to temporarily short-circuit the sensor to make the refrigerator work?
Technically, this is possible for some types of sensors (NTC), if you replace them with a resistor of a certain value that simulates normal temperature. However, this is a temporary and dangerous solution. The refrigerator will lose the ability to automatically regulate, which can lead to either defrosting of food or freezing of the evaporator “to stone.”
Why does the new sensor show the wrong temperature?
There may be several reasons: a defective part, incorrect installation (poor contact with the wall), oxidation of contacts in the connector, or the need for calibration through the service menu. It is also possible that the problem is not in the sensor, but in a freon leak, which is why the temperature physically cannot drop to normal.
How often do you need to change temperature sensors in a refrigerator?
Thermal sensors do not have a strict replacement schedule “by mileage”. They serve until they fail, which can happen after 5 years, or maybe after 15 years of operation. Scheduled replacement is required only if unstable temperature operation is detected.