How to test an NTC thermistor

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.

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.

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.

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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.

John Lewis

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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.

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

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.

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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.

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).

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.

Buy NTC Thermistor

Try 4Washerhelp spares site for Washing machine NTC sensors

How to test an NTC thermistor

What is an NTC thermistor?

John Lewis