Modern refrigerators with the system No Frost have saved us from the need for regular manual defrosting, but have added many electronic components to the design that fail over time. When the unit stops freezing or, conversely, begins to turn food into an icy monolith, most often the culprit is a malfunction of the temperature sensors. These small devices continuously transmit signals to the electronic control module, regulating the cycles of turning on the compressor and the operation of the defrosting heating element.
Understanding how to check the sensor can save you from buying expensive spare parts or calling a technician if the problem lies in banal oxidation of the contacts. Thermistors (this is how these sensors are technically correctly called) change their electrical resistance depending on from the ambient temperature, and it is this parameter that we will measure. If the resistance value is outside the permissible range, the “brain” of the refrigerator receives false data and gives incorrect commands to the actuators.
In this article we will analyze in detail the algorithm of actions, the necessary tools and typical symptoms of malfunction of various sensors in the evaporator area and the fridge compartment. You will learn to distinguish a serviceable element from a defective one using a regular household multimeter, and understand in which cases replacement is really necessary, and when it is enough to simply clean the contacts.
Symptoms of faulty temperature sensors
Before disassembling the equipment and taking up tools, you need to clearly identify the problem. A refrigerator is a complex system, where the same symptom can indicate different breakdowns, however, there are specific signs that are highly likely to point specifically to temperature sensor. In No Frost systems, two types of sensors most often fail: the one that controls the temperature in the evaporator (for defrosting), and the one that monitors the climate in the refrigerator compartment.
If the evaporator defrost sensor fails, the refrigerator can behave extremely strangely. For example, the unit works without interruption, the compressor hums constantly, but the cold does not accumulate, or a huge “fur coat” of ice forms on the back wall of the freezer, which blocks air circulation. In this case, the control module “thinks” that the evaporator is warm and does not start the heating element to defrost.
⚠️ Attention: If you notice that the refrigerator has started to work in increased mode, and it is warm inside the chamber, do not try to immediately change the compressor. In 80% of cases, the problem lies precisely in the sensor circuit or defrost heating element.
If the refrigeration chamber sensor malfunctions, the symptoms will be different. Products in the main compartment can either freeze (if the sensor “lies” that it is warm there and the refrigerator does not turn off), or, conversely, spoil due to insufficient cold. Also, the emergency temperature indicator on the control panel may light up, even if visually everything is in order with the food.
- 🧊 Ice freezing on the back wall of the freezer, blocking the ventilation holes.
- 🔊 The compressor runs continuously, without stopping cycles, or, conversely, turns on too rarely.
- 🌡️ The temperature in the chambers does not correspond to the set values on the display or controller.
- 💡 Flashing of the “Alarm” or “Attention” indicator on the front panel of the refrigerator.
Required tools and preparation
For high-quality diagnostics you will need a minimum set tools that most home craftsmen have. The main device will be multimeter (tester), capable of measuring resistance in Ohms. Without this device, checking will be impossible, since visually determining the serviceability of the thermistor is almost impossible - it looks like an ordinary plastic or metal capsule with two wires.
You will also need screwdrivers (Phillips and flathead) to dismantle the panels, a knife to carefully cut off the insulation if necessary, and electrical tape for subsequent protection of the connections. A hairdryer will not be superfluous if there is a need to artificially change the temperature of the sensor to check its reaction, although at home they often use just the warmth of their hands or a container of warm water.
Before starting any work on disassembling and checking electrical circuits, it is necessary to turn off the power to the refrigerator. Unplug the power cord from the outlet. This is a critically important safety rule, since you will touch elements that are energized while the compressor or heating element is operating.
☑️ Preparing for diagnostics
It is also important to prepare the workplace. Clear some space around the refrigerator to allow access to the rear of the unit. If the model is built-in, it will have to be removed from the niche. Take care in advance of good lighting, as sensors are often located in hard-to-reach places with poor visibility.
Location of sensors in the No Frost system
To check the element, you need to know where it is located. Refrigerators with the No Frost system usually have several sensors installed, and their number depends on the model and manufacturer. The main installation points are the evaporator area (in the freezer) and the interior of the refrigerator compartment.
The evaporator defrost sensor is most often mounted directly on the evaporator tubes or inserted into a special hole in the plastic housing of the air duct. To get to it, in most models you will have to remove the back panel inside the freezer, and sometimes even remove the evaporator itself. It looks like a small capsule, often white or black, with two matching wires.
⚠️ Attention: The design of refrigerators from different brands (Indesit, Samsung, LG, Bosch) may vary greatly. In some models, access to the sensor requires complete defrosting of the unit within 24 hours, so as not to damage the fragile plastic during dismantling.
The refrigeration chamber sensor is usually located on one of the side walls inside the compartment, sometimes hidden behind a decorative trim or located in the area of the lampshade. In two-chamber models there can be two of them: one main and one additional for the “zero temperature” or freshness zone.
Hidden installation of sensors
In some models of Samsung and LG refrigerators, the sensor can be filled with foam or hidden deep in the air duct housing. In such cases, to replace it, you need to carefully foam the polyurethane foam or partially disassemble the plastic box, which requires increased care.
For clarity, let's look at the typical arrangement of components in the table:
| Sensor type | Typical arrangement | Function |
|---|---|---|
| Defroster (Defrost) | On the evaporator tubes, inside the freezer | Temperature control to turn on the heating element |
| Refrigerator compartment | On the side wall or at the top of the compartment | Adjusting the compressor operation |
| Cut-offs (optional) | In the evaporator area | Emergency shutdown of the heating element when overheated |
Step-by-step instructions: how to ring a sensor
The most reliable diagnostic method is resistance measurement. To do this, switch the multimeter to resistance measurement mode (indicated by the Ω icon), selecting the limit of 20 kOhm (20,000 Ohm). If your tester does not have an automatic limit selection, start with the maximum and gradually reduce it to get an accurate reading.
Remove the sensor from its seat and disconnect it from the main wiring of the refrigerator. They are often connected via a connector, but in some budget models the wires may be twisted and insulated. Carefully reach the contacts of the sensor itself. Place the multimeter probes on the sensor contacts. Polarity is not important here, since the thermistor is a passive element.
Record the readings on the screen. At room temperature (about +20...+25°C), a working sensor usually shows a resistance in the range from 4 to 7 kOhm (specific numbers depend on the model, but “open” or “zero” is definitely a malfunction). If “1” or “OL” lights up on the screen (in different ways on different devices), this means an open circuit - the sensor is faulty.
For a more accurate check, you can conduct a temperature change test. Hold the sensor in a warm palm or lower it (without immersing the contacts!) in warm water. The resistance should start to change (usually decrease as it gets warmer). If the numbers on the screen stay the same, regardless of the temperature, the element is defective.
- 🔌 Disconnect the sensor connector from the refrigerator wiring harness.
- 📟 Connect the multimeter probes to the sensor contacts.
- 🌡️ Compare the readings with the reference values for your model.
- 🔄 Check the resistance response to heating or cooling.
Table of typical resistance values
Although there is no universal figure, since each manufacturer uses its own components, you can focus on the average values for the most common brands. Knowing these parameters will help you make a quicker decision about replacement.
It is important to understand that resistance depends on temperature. The data below is valid for an ambient temperature of approximately +25°C. If you take measurements in a cold garage or in the cold, the readings will be significantly higher.
| Refrigerator brand | Sensor type | Resistance at +25°C | Resistance at 0°C |
|---|---|---|---|
| Indesit / Ariston | Defroster (defrost) | ~ 6.6 - 7.2 kOhm | ~ 13 - 15 kOhm |
| Samsung | Refrigerator compartment | ~ 5.0 - 5.5 kOhm | ~ 10 - 12 kOhm |
| LG | Evaporator | ~ 5.0 kOhm | ~ 9 - 10 kOhm |
| Bosch / Siemens | Universal | ~ 2.0 - 2.5 kOhm | ~ 4.5 - 5.0 kOhm |
If your measurements differ radically from the tabular data (for example, it shows 0.5 kOhm or 50 kOhm at room temperature), most likely the thermistor needs to be replaced. Also pay attention to the integrity of the wire insulation: cracks and creases can lead to periodic failures.
⚠️ Attention: Technical characteristics and diagrams may change in different modifications of the same model. Always check the official documentation or the markings on the sensor itself, if they are readable.
Replacement and possible errors when installation
If the diagnostics showed a malfunction, the sensor must be replaced. It is better to buy an original spare part or a high-quality analogue with identical resistance parameters. Installation of a new element is carried out in the reverse order.
Pay special attention to the tightness of the wire connection. There is high humidity and constant temperature changes in the evaporator area. Use heat shrink and, preferably, moisture-resistant sealant to insulate the twists or solders. Over time, ordinary electrical tape can peel off from condensation, which will lead to a short circuit.
A common mistake is incorrect installation of the sensor in the mounting location. It must fit tightly to the surface it controls (evaporator tube or chamber wall). If there is an air gap between the sensor and the surface, the temperature readings will be distorted and the refrigerator will continue to operate incorrectly, even with a new sensor.
After replacement, assemble the refrigerator, plug it in and listen to the operation. After 15-20 minutes the compressor should start. Full restoration of the temperature can take from 2 to 6 hours, depending on the volume of the chambers and the number of loaded products.
What to do if the new sensor did not help?
If replacing the sensor did not solve the problem, the reason may lie in the control module itself (electronic board), which processes signals incorrectly, or in a broken wiring between the sensor and the board. It is also worth checking the defrost heating element and the timer (if it is mechanical).
Is it possible to temporarily short-circuit the sensor?
Theoretically, short-circuiting some types of sensors can make the refrigerator work bypassing the automation, but this is dangerous. The unit may go into continuous operation, which will lead to freezing of the evaporator or failure of the compressor. This method is only suitable for short-term diagnostics.
Why do sensors often fail?
The main reason is constant heating and cooling cycles that destroy the internal structure of the semiconductor, as well as moisture penetrating inside the sensor housing through microcracks in the wire insulation.