A modern refrigerator is a complex electromechanical device where every the element plays a key role in maintaining temperature conditions. One of the most common causes of system failure No Frost is the failure of the automatic defrosting unit, the central element of which is the PZR (posistor). When this small component stops functioning, the evaporator begins to form a “coat” of ice, which stops the circulation of cold air and spoils food.
Owners often notice the problem too late, when the refrigerator stops freezing or starts making strange sounds. Understanding how to check the EPR of a refrigeratorallows you to independently diagnose the malfunction before calling a technician or purchasing expensive equipment. In this article, we will analyze the physical principle of operation of a posistor, its role in the control circuit and a step-by-step algorithm for testing with a multimeter.
Diagnostics takes a little time, but requires compliance with safety precautions and a minimum set of tools. If you are willing to carefully disassemble the back wall of the freezer and hold the tester in your hands, you can save a significant amount on service costs. Let's figure out why this element is so important and what exactly happens inside the electrical circuit when it breaks down.
The role of a posistor in an automatic defrosting system
A posistor, or a thermistor with a positive temperature coefficient of resistance (PTC), performs the function of a thermal relay in the evaporator heater circuit. Unlike a bimetallic thermostat, which opens the contacts mechanically, the PZR changes its electrical resistance depending on the temperature. In a cold state, it has low resistance and passes current to Heating element (tubular electric heater).
As soon as the heater starts working, the temperature of the posistor itself increases. When a certain threshold is reached (usually around 60–80 degrees Celsius), its resistance sharply increases thousands of times, effectively breaking the circuit and turning off the heating. This process prevents the evaporator from overheating and the plastic parts of the chamber from melting.
⚠️ Attention: The posistor is a disposable element in most household refrigerators. After cooling, it does not return to its original state with low resistance instantly or does not return at all, so its reuse after operation is often impossible.
It is important to understand that the PZR works in tandem with a timer or electronic control module. The command to turn on is given by the “brain” of the refrigerator, and the posistor ensures the safe completion of the heating cycle. If this element is “stuck” in a state of high resistance, the heater will never turn on, and the ice on the evaporator will build up with each cycle.
Symptoms of a malfunction: when diagnostics are needed
You can determine that the problem lies in the defrost system by a number of indirect signs. Users often confuse them with a compressor failure or freon leak, which leads to incorrect diagnosis. The first and most obvious sign is the formation of a block of ice on the back wall of the freezer, which can block the air supply channels.
The refrigerator may work in increased mode, trying to gain temperature, but due to the evaporator blocked by ice, cold air simply does not enter the main chamber. As a result, the food in the refrigerator compartment becomes warmer, and the temperature in the freezer may be unstable.
Pay attention to the following symptoms:
- 🧊 A thick layer of ice or a “snow coat” is visible on the back wall of the freezer, although this has not been observed before.
- 🔇 The compressor is working almost without stopping, trying to compensate for the lack of cold, which leads to increased energy consumption.
- 💧 Water appears under the refrigerator or inside the chamber, which does not have time to evaporate due to a violation of the defrosting cycle.
- 🔊 The appearance of extraneous crackles or clicks in the area of the evaporator during operation of the unit.
If you observe at least two of the listed signs, the probability of failure defrost sensor or the heating element is extremely high. Before sinning on the compressor, you need to check the defrost circuit, where the PZR plays a key role.
Preparation for the test: tools and safety
Before you start directly checking the PZR, you need to properly prepare the workplace and tools. Working with electrical appliances requires strict adherence to electrical safety rules. Any manipulations inside the case are possible only after completely disconnecting the device from the electrical network.
You will need a standard home craftsman's kit, which most men have. The main tool will be multimeter (tester), capable of measuring resistance in the range from 0 to 20 kOhm. Without this device, high-quality diagnostics is impossible, since it is impossible to determine the integrity of the posistor “by eye.”
List of necessary equipment:
- 🔌 Multimeter with working probes and a working battery.
- 🪛 Set of screwdrivers (phillips and flat) for dismantling panels.
- ✂️ Knife or flat spatula for carefully removing plastic latches.
- 🔦 Flashlight for illuminating hard-to-reach places behind the back wall.
⚠️ Attention: Do not measure resistance in a live circuit. This is guaranteed to lead to the burnout of the multimeter fuses or failure of the device.
It is also recommended to prepare a container in advance to collect melt water if you decide to defrost the refrigerator before checking. Although complete defrosting is not always necessary to diagnose PZR, the presence of moisture can create a risk of short circuit if accidentally turned on.
Access to the sensor: removing the rear panel
To get to the posistor, you need to remove the back panel in the freezer. The design of refrigerators from different brands (Indesit, LG, Stinol, Beko) may differ, but the general principle is similar. First, remove all drawers and shelves, freeing up space.
The back wall is usually secured with screws or plastic latches. In some models, the screws may be hidden under decorative plugs. Act carefully, plastic becomes brittle in the cold and easily breaks if tugged carelessly.
☑️ Removing the evaporator panel
After removing the panel, you will see the evaporator - a radiator with many thin plates. At the bottom of the evaporator, often under an aluminum groove or in a special recess, the heating element itself is located posistor. It is a small black or white plastic cylinder with two or three wires coming to it.
Visually inspect the element. If traces of burning, melting or mechanical damage are visible on the body, further inspection may not be necessary - the part requires replacement. However, the absence of external defects does not guarantee serviceability, so we move on to electrical measurements.
Algorithm for checking PZR with a multimeter
The most reliable way to check is to measure electrical resistance. To do this, switch the multimeter to resistance measurement mode (indicated by the Ω symbol), selecting the 20 kOhm limit. If your tester has a “continuity” mode, it is also suitable for initial diagnostics.
Disconnect the connector with wires from the posistor. In some models, the wires may be twisted or soldered - in this case, they must be carefully disconnected to prevent the rest of the refrigerator circuit from influencing the readings of the device. Apply the multimeter probes to the PZR contacts.
Interpretation of the readings:
- 📉 Low resistance (100–800 Ohms): The posistor is operational and is in a cold state, ready to pass current to the heating element.
- ♾️ Infinity (one on the screen): Open circuit. The posistor has burned out and does not conduct current. Replacement is required.
- 📈 High resistance (several kOhms and higher): The element has degraded or is “stuck” in the open state. It will not turn on the heating.
Nuance of checking three-pin posistors
Some circuits use posistors with three terminals. Typically two of them are the main circuit, and the third may be connected to the control system or have another purpose. Check the resistance between each pair of contacts. The operating resistance between the main contacts should be low.
It is worth noting that the “cold” test only shows the integrity of the circuit. To be 100% sure, some technicians heat the element with a hairdryer while measuring. When heated, the resistance of a serviceable PZR should increase sharply. However, in everyday conditions it is enough to make sure that in a cold state it conducts current and is visually free of damage.
Table of typical resistance values
For ease of diagnosis, we provide reference data. The values may differ slightly depending on the component manufacturer (Klixon, Defrost etc.), but the order of magnitude remains similar.
| PZR state | Temperature | Expected resistance | Action |
|---|---|---|---|
| Cold (serviceable) | ~20°C | 100 - 800 Ohm | Normal, can be used |
| Heated (serviceable) | > 60°C | > 10 kOhm (Sharp increase) | Normal, circuit broken |
| Burnt | Any | ∞ (Infinity) | Replacement is required |
| Closed | Any | 0 Ohm | Critical fault |
If your measurements match the line “Burnt” or “Short-circuited”, buying a new part is a matter of time. They are inexpensive, but their absence can lead to failure of a more expensive compressor due to constant operation.
Replacement and testing of the rest of the system
If the check shows a faulty PZR, it must be replaced with a similar one. When purchasing, pay attention not only to the appearance, but also to the labeling. The important parameters are: nominal resistance, operating current and geometric dimensions so that the part fits into its normal place.
However, when replacing a posistor, it would be useful to check other elements of the circuit. Often the cause of a breakdown is not only the sensor itself, but also Defrosting heating element or the fuse. If the heating element is broken, then even a new posistor will not start the defrosting process.
⚠️ Attention: When assembling, make sure that the wires of the posistor and heating element do not touch the metal parts of the evaporator or sharp plastic edges. Over time, the vibration of the compressor can grind the insulation, which will lead to a short circuit.
After replacement, reassemble everything in the reverse order, but do not reinstall the back panel yet. Turn on the refrigerator and run a test cycle (if your model allows forced defrosting through the menu) or simply wait for the timer to go off. After 10–15 minutes of operation in the defrost mode (if it occurs), the new PZR should heat up and turn off the heating element.
Frequent errors in diagnostics
Beginner repairmen often make mistakes that lead to false conclusions. For example, they try to check the PZR without disconnecting it from the general circuit of the refrigerator. In this case, the multimeter may show the resistance of parallel circuits, giving a false positive result.
Another mistake is ignoring the condition of the wiring. Oxidized contacts in the connector can produce high resistance, which the user mistakenly attributes to the posistor itself. Always clean the contacts before measurements.
Typical errors:
- 🚫 Checking without disconnecting the connector from the control board.
- 🚫 Ignoring visual inspection of wires for fractures.
- 🚫 Using a faulty one multimeter with a dead battery.
Frequently asked questions (FAQ)
Is it possible to temporarily operate a refrigerator with a faulty PZR?
Theoretically, the refrigerator will freeze, but only until the evaporator is completely covered ice. This may take from 3 days to 2 weeks. After this, the cold will stop entering the chamber. Long-term operation in this mode is harmful for the compressor, which will work without interruption.
How does the PZR differ from a defrost thermostat?
The thermostat (bimetallic) opens the circuit mechanically at a certain temperature and closes it again when cooling. The PZR changes resistance and often requires replacement after each heating cycle, since it does not always return to its original state with low resistance.
Where to buy a suitable posistor?
These parts are sold in spare parts stores for household appliances, on radio markets and online platforms. It is important to know the model of your refrigerator or the parameters of the burnt element (usually indicated on the body).
Why does a new PZR burn out immediately after installation?
This may indicate a malfunction of the defrost timer (it does not turn off the heating on time) or problems with the control module. Also, the cause may be a short circuit in the heating element itself.