High Lockout
F8 on Saunier Duval Isotwin F30E

Your boiler's temperature sensor for the hot water storage tank has developed a fault, meaning the system cannot safely monitor or heat your hot water.

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.

Technical description

Tank thermistor fault

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature in your hot water storage tank fails to send a reliable electrical signal to the boiler’s control board. The most common reason is the internal degradation of the thermistor or a loose connection in the wiring, which prevents the boiler from knowing how hot the water is. For safety reasons, the system shuts down hot water production to prevent potential overheating.

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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Related fault codes

F1View →F2View →F6View →

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