BoilerFaultCodesYour boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, usually causing the system to shut down as a safety precaution.
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
What causes this fault?
This fault occurs when the internal sensor responsible for measuring the water temperature as it leaves the boiler fails or provides an unrealistic reading. The most common reason is that the sensor has physically degraded over time or a loose electrical connection has developed, preventing the control board from safely monitoring the heat levels.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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A boiler cover plan would cover faults like F04 L04 — 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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