High Lockout
F5 on Ideal Logic Max System

Your boiler has detected an issue with the sensor that monitors 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 stops receiving a reliable signal from the sensor that tracks the temperature of the water coming back from your heating system. The most common reason is that the sensor has physically failed or developed a loose connection, preventing the boiler from safely monitoring heat levels.

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 →F2View →L2View →

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