High Lockout
F5 on Keston S30

Your boiler has stopped working because it cannot properly measure the temperature of the water coming back from your radiators.

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

Return thermistor

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler begins sending incorrect data or fails completely. The most common reason is that the internal components of the sensor have degraded over time, though it can also be caused by loose electrical connections or a build-up of debris on the sensor's surface.

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

F4View →L2View →

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