BoilerFaultCodesYour 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.
Book a Gas Safe engineer within 24–48 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 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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