High Lockout
F7 on Saunier Duval Isotwin F30E

Your boiler has detected a problem with the sensor that monitors hot water temperature, which typically means you will have little or no 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

DHW thermistor fault

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors your hot water temperature. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be triggered by loose or corroded electrical connections between the sensor and the main control board.

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 →F13View →

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