High Lockout
F4 on Ideal Logic+ Heat

Your boiler has stopped working because it cannot accurately measure the temperature of the water flowing through the system.

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

Flow thermistor fault

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water leaving the unit. The most common reason is that the sensor has physically failed or developed a chemical 'drift,' meaning it can no longer provide a reliable reading to keep the system running safely.

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

F3View →F5View →L2View →

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