High Lockout
F5 on Ideal Logic+ System

Your boiler has stopped working because it cannot properly measure the temperature of the water returning 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 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 coming back from your heating system. The most common reason is that the sensor itself has suffered an internal electrical failure, though it can also be caused by loose wiring or a damp connection inside the boiler.

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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