High Lockout
F4 on Ideal Logic Combi C24 C30 C35

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely regulate heat.

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 being sent to your taps and radiators. The most common reason is that the sensor has physically failed or developed an electrical short, though loose or damaged internal wiring can also prevent the boiler from reading the temperature correctly.

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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