High Lockout
F4 on Ideal Logic+ System

Your boiler has stopped working because it cannot correctly detect how much water is flowing through the system or what the temperature is.

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

Control / no flow thermistor fault

What causes this fault?

This fault occurs when the boiler's internal sensors provide conflicting or impossible temperature readings, making it unsafe for the unit to continue heating. The most common cause is a faulty flow thermistor that has failed electrically, or a significant lack of water circulation preventing the heat from moving away from the heat exchanger. In some cases, a communication error on the main circuit board can lead the boiler to incorrectly report this flow issue.

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 →L1View →L2View →

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