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Boiler NTC thermistor (temperature sensor): What It Does and Signs It's Failing

Boilers measure water temperature with small NTC thermistors clipped or screwed onto the pipes inside the casing. There's usually one on the flow pipe leaving the heat exchanger and one on the return, plus one on the hot water side of a combi. A system boiler can also have a sensor in the hot water cylinder.

How it works

NTC means the sensor's resistance falls as it gets hotter. The board reads that resistance and converts it to a temperature; Ideal's sensors read about 10 kΩ at 25°C (77°F). An open circuit, a short or a value that doesn't change when the water heats gets flagged as a fault.

Signs it is failing

• Temperature sensor or short-circuit code
• Boiler shutting down soon after lighting with no other symptom
• Water getting hotter or cooler than the setting
• Hot water temperature that wanders on a combi

Sensor open-circuit, short-circuit, overheat and flow-return gap codes all involve these sensors and their wiring. The fault often turns out to be a loose plug on the harness.

Can you replace it yourself?

Sensors sit inside the casing next to live wiring, so testing and replacing them is a job for a Gas Safe engineer. Don't try meter testing yourself. If the code mentions a cylinder sensor on a system boiler, tell the engineer when you call.

Buying the right part

Clip-on and immersion types aren't interchangeable, and resistance curves vary by brand, so use the sensor listed for the boiler model.

Error codes linked to the ntc thermistor (temperature sensor)

• E110 / E130Overheat lockout (primary water or flue)Baxi
• F.20Overheat safety cut-out has trippedVaillant
• F.23Too big a gap between flow and return temperatureVaillant
• F.00Flow or return temperature sensor not readingVaillant
• F.10Temperature sensor short circuitVaillant
• F.13Hot water cylinder sensor short circuitVaillant
• F.83Dry fire: no temperature change when the burner litVaillant
• E9 / E9 224Overheat limiter trippedWorcester Bosch
• E2Heating flow temperature sensor faultWorcester Bosch
• A7Hot water temperature sensor faultWorcester Bosch

Last reviewed 2026-10-21.