High Lockout
F4 on Ideal Logic Combi ESP1

Your boiler has likely stopped heating because a sensor that measures the temperature of the water leaving the unit has failed or become disconnected.

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 responsible for monitoring the temperature of the water sent to your radiators. The most common reason is that the sensor has suffered an internal electrical failure or the wiring connecting it to the main circuit board has become damaged or loose.

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

F2View →F3View →L2View →

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