High Lockout
F4 on Ideal Logic Max System

Your boiler has detected an issue with the sensor that measures the temperature of the water leaving the unit, preventing it from heating your home safely.

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 internal sensor responsible for monitoring the heat of the water leaving the boiler fails to send a clear signal back to the control board. The most common reason is that the sensor has physically degraded or developed an electrical short, meaning the boiler can no longer safely tell if the water is getting too hot. In some cases, loose wiring or a connection issue between the sensor and the main computer can also trigger this shutdown.

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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