High
F6 on Saunier Duval Isotwin F30E

Your boiler has detected a problem with the sensor that monitors the temperature of the water heading to your radiators, causing the system to shut down for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

What the manual says

Manufacturer manual

Central heating flow thermistor fault. Thermistor cable defective/broken. Thermistor faulty. Check that thermistor attached correctly to pipe

Shared across the range
This code is shared by the boilers in this range, which use the same control board. It may not be printed in this exact variant’s manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

What causes this fault?

This fault occurs when the boiler's internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the water leaving the unit. The most common reason is that the sensor has physically failed due to age, or the electrical connection between the sensor and the main control board has become loose or damaged.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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