High Lockout
F12 on Glow-worm Ultracom hxi

Your boiler has lost communication with the sensor that monitors the temperature of your hot water cylinder, meaning it cannot heat your water correctly.

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

Cylinder thermistor fault

What causes this fault?

This fault occurs when the boiler can no longer detect a signal from the sensor that monitors the temperature of your hot water tank. The most common reasons are a breakdown of the internal sensor components or a loose connection in the wiring that links the sensor to the boiler's 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 →F11View →F13View →

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