BoilerFaultCodesYour boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely control the heat.
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 boiler's internal computer stops receiving a reliable signal from the sensor that monitors the temperature of the water being sent to your taps or radiators. The most common reason is that the sensor itself has developed a physical defect or the electrical wiring connecting it to the main control board has become damaged or loose. Without this data, the boiler shuts down as a safety precaution to prevents it from potentially overheating.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F 04 OR L 04 — 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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