How to test an NTC thermistor

How to If you want to test an NTC thermistor (or sensor) from a washing machine or other white goods appliance this is the right article for you. It also describes what a thermistor is, what it does, and what commonly goes wrong with them.

What is an NTC thermistor?

A thermistor is essentially a resistor that changes its electrical resistance at varying temperatures. The name is derived from a mix of thermal and resistor. They are used in washing machines, tumble dryers and even fridges and freezers as thermostats or temperature sensitive safety cut out devices. Because their electrical resistance at various temperatures is known, it’s easy for the appliance to monitor a thermistor and detect when certain temperatures are reached.

Thermistor This picture shows a thermistor from an AEG washing machine. It’s just slightly larger than a £2 coin. However, thermistors do not all look the same and can come in different shapes and sizes – they are most commonly small and round though.

Indesit washing machines used to use long thin thermistors inserted into the heating element. The main concern is correctly identifying a thermistor because they look very similar to TOCs (thermal overload cut-outs). Both have just 2 wires and are usually small and round. A thermistor may have temperature ranges printed on it but many are too small to fit much on them. A TOC (which is a thermal fuse) may have a temperature that it trips out at printed on it.

A TOC has a direct connection inside with no resistance (or open circuit if it’s blown), but a thermistor will have high resistance and that resistance will vary depending on the temperature. It’s possible for one to erroneously appear open circuit if you don’t have the range correctly set to high on a continuity test meter.

How to test an NTC thermistor

Unless you can see a wire has come loose you are unlikely to see anything physically wrong. Diagnosis is done by measuring the resistance with a continuity (or multi) meter. I can’t give readings for all thermistors because they vary, especially in different types of appliance, but you should be able to get a reading across both terminals. If it’s completely open circuit it’s likely to be faulty.

Readings on a washing machine’s NTC thermistor for example may be something like 12000 Ohm at 25°C or 3243 Ohm at 60 ° C. The resistance at 0 ° C may be more like 36000 Ohm. However, it may be possible for readings to be present but incorrect.

For example, the resistance figures I can see in an old Hotpoint washing machine manual qualify the resistance readings by saying plus or minus 6% for the resistance at 0° down to plus or minus 2% for the resistance at 60° implying that readings outside these parameters are incorrect. Therefore without the exact reading expected for any individual thermistor you can’t be sure if it is correct or not.

In an old Hoover manual they say their washing machine NTC should have the following resistance readings – 20K at 25 ° C and 2.14K at 85° C. So NTC’s vary, but they should usually have resistance readings that change at different temperatures.

If you get no resistance reading at all (open circuit) then it’s probably faulty but you should at least check it at different temperatures by placing one in a saucer of warm water for example and carefully adding freshly boiled. The hotter the temperature the less resistance should be measured.

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What does NTC stand for?

NTC stands for Negative Temperature Coefficient which basically means that their electrical resistance decreases with increasing temperature.

What goes wrong with thermistors?

Connection faults where the wires connect inside, breaks within the wires leading to and from the thermistor (it’s possible for some to be invisible i.e. the outer plastic seems ok but the wire inside is broken). Other than that the thermistor may stop giving reliable readings. Normally though, a faulty thermistor should trigger some error code.

Symptoms of a faulty thermistor

If a thermistor goes faulty it could either fail to give any temperature readings or give incorrect ones. If it no longer registers any temperature change the appliance should overheat if it’s used in conjunction with heating elements, or if used in refrigeration it could cause temperatures to get too low. In most cases however, appliances using these devices are sophisticated enough to recognise the sensor isn’t working and produce some error code.

Faulty thermistor in washing machines

Error codes should be produced, though unfortunately they won’t be identified in the instruction book so you won’t know what they are referring to unless you can find reference somewhere else. These codes vary across brands and even models so impossible to list. Some examples of error codes implicating thermistors are here Hotpoint WD washer dryer error code F03 | Indesit washing machine flashing 3 times – error code F03 (These two codes are both F03 because the brands are manufactured by the same company).

Filling and emptying at same time

One symptom of a washing machine that thinks it’s overheating (possibly because of a faulty thermistor) is if it’s filling with cold water and pumping out at the same time. This is covered in this article Washing machine fills and drains at same time although the same symptom can be caused by other faults.

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4 thoughts on “How to test an NTC thermistor”

  1. Hi…Thanks for the very useful info . I have a Bosch Washer/Dryer WVT-1260 that didn’t dry properly. I read that on a drying cycle the heater does not activate for the first 10 minutes. What is the reason for this? I verified this with a multimeter across the heating element when a 10 minute dry time was selected and the heater wasn’t energised at all. Selecting a 30 minute time caused the heater to energise approx 90 seconds after pressing start. I don’t understand this. Any ideas?
    Regarding your article ” How to test an NTC thermistor”. It wasn’t until I printed the pages that I realised that the article was arranged in two columns and not in a more conventional left to right convention. Throw in the odd advert ie Forbes Rentals…and it all adds confusion. Why do this?

  2. Andy Trigg (Whitegoodshelp)

    Hello Gregory. Thank you for your comment. Washerdryer’s and tumble dryers turn the heating element off in the last 10 minutes, and not the first 10 minutes. So if you set it for 30 minutes it will heat for 20 minutes and turn the element off in the final 10 minutes. This is to facilitate a gradual cooldown of the laundry otherwise when the drying cycle finished the laundry would be red-hot, too hot to handle, and also they would become badly creased.

    Regarding the layout, the vast majority of people apparently view my site on a mobile device. When doing so each paragraph is presented below the other which is great. But on a computer screen the layout is similar to a newspaper or magazine with two or three columns. The horizontal lines indicate that each section is separate, so you read the first section from left to right and then drop down to the next section and read from left to right again. I appreciate that a newspaper or magazine you normally follow the first column right down to the bottom and then go to the next column that on the computer screen that would involve a lot of scrolling up and down and would be very inconvenient.

    I do appreciate it is a bit unconventional and I’ll be hundred percent honest I’ve not been totally happy with it myself and have felt it is the best compromise but I will look into improving the layout even further. Thank you for your feedback.

  3. Johnny Whitright

    Does anyone know the proper resistance scale for the AEG 132574300 thermistor? I have a Zoppas (Electrolux) washing machine that makes very hot water when set at 90 degrees (Celsius) but when set at 60 degrees it seems more like 40. At about 18C (65F) the thermistor shows about 6600 ohms resistance. At 37C (98.6F) it shows about 3300. Does this sound right?

  4. Andy Trigg (Whitegoodshelp)

    Hello Johnny. All I have is a few examples of resistance is in a different brand but presumably I wouldn’t be surprised if they are all virtually the same as they all do the exact same job. Have you checked that the temperatures are actually incorrect with a thermometer?

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