BoilerFaultCodesYour boiler has stopped working because it cannot correctly detect how much water is flowing through the system or what the temperature is.
Book a Gas Safe engineer within 24–48 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 fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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A boiler cover plan would cover faults like F4 — no call-out fees, no surprises.
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Could it be time for a new boiler?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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