A modern refrigerator is complex an electronic-mechanical system where each element performs a strictly assigned role. One of the key parts in the defrost circuit is evaporator sensorwhich controls the temperature on the surface of the cooling radiator. If this component fails, the unit stops correctly switching to defrost mode or, conversely, begins to heat the evaporator too often and for a long time.
Owners are often faced with a situation where a “snow coat” appears in the chamber, and ice builds up on the back wall, blocking the air channels. This is a direct consequence of the fact that thermistor (the second name of the sensor) transmits incorrect data to the control module. As a result, the compressor wears out, and the products begin to deteriorate due to temperature violations.
Before calling a technician or buying a new part, you need to carry out independent diagnostics. The critical parameter when checking is the exact value of the resistance at a specific ambient temperature, since even a small deviation can throw off the logic of the controller. In this article we will look at how to identify a breakdown, what tools will be needed and how to interpret the received data measurements.
Symptoms of a malfunction of the evaporator temperature sensor
Understanding how the refrigerator behaves when the sensor breaks down allows you to narrow down the search for the problem. Most often, the malfunction does not appear immediately after switching on, but after several cycles of operation, when the defrost system should have worked, but was not activated due to erroneous readings.
One of the main signs is the formation of a dense layer of ice on the back wall of the freezer or on the evaporator itself, which is visible through the grille. If ice builds up enough to interfere with the movement of the damper or fan, the system stops cooling effectively and the temperature in the main chamber begins to rise.
- ❄️ A thick layer of ice or snow has formed on the back wall of the refrigerator or freezer.
- 🌡️ The temperature in the compartments is not correct set values, although the compressor operates continuously.
- 🔊 You can hear the crackling sound of breaking ice or the hum of a fan that touches the ice crust.
- 💡 The emergency temperature indication lights up on the refrigerator display or the defrost indicator is blinking.
It is also worth paying attention to the operating mode compressor. If thermostat or the electronic module receives a signal that the evaporator is warm (when in fact it is cold), it may not turn on the defrost heater. As a result, the refrigerator works in increased mode, trying to compensate for poor heat transfer through the layer of ice.
⚠️ Attention: If you notice that the refrigerator has started to work cyclically with very short pauses, this may indicate that the sensor is “lying” and causes the unit to constantly try to gain temperature.
In some models, for example Indesit or Atlant, the malfunction may not be displayed as an error code immediately, but appear only after a week or two of operation. While more complex systems, such as LG or Samsung, often immediately display a fault code on the screen, indicating a problem in the sensor circuit.
Where the sensor is located and how to get to it
The location of the sensor depends on the design of the refrigerator and the type of cooling system - this is a drip system or No Frost. In most cases, the sensor is attached directly to the evaporator tubes or inserted into a special hole in the aluminum radiator.
To access the part in refrigerators with the system No Frost you will need to completely defrost the unit and remove the back panel of the freezer. It is there, hidden behind the plastic casing, that the evaporator is located with tubes wound around it and a sensor attached.
The nuances of location in different brands
In Bosch refrigerators, the sensor is often pressed into the evaporator tube and requires replacement along with a section of the pipeline, while in Beko models it is usually attached with a clip and is easily is removable.
Unplug the cord from the outlet and leave the doors open for at least 2-3 hours so that all the ice melts. Trying to chip the ice with a knife can lead to tube breakdown and freon leakage, which will become a much more serious and expensive problem.
After removing the panel, you will see an aluminum radiator. The sensor is a small capsule (often black or white) from which two wires extend into the wiring harness. It can be glued with thermal paste or clamped with a metal bracket.
- 🔍 Remove the protective plastic panel of the freezer by unscrewing the mounting screws.
- 🧊 Make sure that there is no ice on the evaporator that prevents safe access to the wires.
- 🔌 Carefully disconnect the sensor connector from the main wiring or cut the wires (if you plan to solder a new one).
Access to the sensor in refrigerators with a weeping system is usually easier - it can be located on the back wall of the refrigerator compartment, but more often in such models one common thermostat is used, rather than a separate defrost sensor. However, in modern two-compressor models, defrost sensors are also present in the freezer.
Necessary tools for diagnostics
For a quality check of serviceability thermistor a visual inspection alone is not enough. You will need a special measuring device that can determine the resistance of the circuit. Without it, diagnostics will be nothing more than a thermistor
The main tool will be multimeter (tester). It must be able to switch to resistance measurement mode (Ohm). It is advisable to have a device with the ability to test the circuit to quickly check for an open circuit.
☑️ Tools for checking the sensor
You may also need a container with water at room temperature or, conversely, a container with ice to place the sensor there and check the change his readings over time. If you want to carry out professional diagnostics, checking the readings with a reference thermometer will not be superfluous.
To remove the old sensor and install a new one, you may need side cutters, a soldering iron, solder and heat-shrink tubing if you plan to connect the wires not through connectors, but by twisting and soldering. The female-male terminals are also useful for restoring contact.
⚠️ Attention: Do not use an open flame (lighter or burner) to heat the sensor. Sudden and excessive heating can damage the internal structure of the thermistor even before the test is completed.
It is important to prepare a workplace with good lighting. The sensor wires are thin and fragile and can be easily damaged by careless movement. If you plan to unsolder the sensor from the control board (which is rare, usually the sensor itself is checked), you will need a desoldering pump.
Step-by-step instructions: how to check the sensor with a multimeter
The testing process is based on measuring electrical resistance. A good sensor is a negative temperature coefficient (NTC) thermistor. This means that when the temperature drops, its resistance increases, and when it heats up, it decreases.
First switch the multimeter to the resistance measurement mode to the limit of 20 kOhm (20,000 Ohm). If your sensor is rated at 5 or 10 kOhm, this limit will be sufficient. Touch the sensor contacts with the probes. If there is “1” or “OL” on the screen, then there is an open circuit.
For a more accurate test, it is necessary to place the sensor in an environment with a known temperature. Place it in a glass of water at room temperature (about +20..+25°C). Record your readings. Then you can add a little ice to the water to cool it to +5..+10°C, and take measurements again.
The resistance should change smoothly. If, when immersed in cold water, the numbers on the display do not change or jump chaotically, the part is faulty. Also check the wires along their entire length for microcracks, especially at the point where they exit the sensor housing.
| Environment temperature (°C) | Resistance (kOhm) for 5k sensor | Resistance (kOhm) for a 10k sensor | Resistance (kOhm) for a 50k sensor |
|---|---|---|---|
| +25 °C | ~5.0 kOhm | ~10.0 kOhm |