BoilerFaultCodesYour boiler has detected that water isn't moving through the system fast enough, causing a large temperature difference that has triggered a safety shutdown.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Water shortage, temperature spread between flow and return probe too large: Pump blocked or defective, pump output too low, flow and return sensor swapped over
What causes this fault?
This fault occurs when the water inside your boiler heats up much faster than it can be moved away into your radiators. The most common reason is a failing internal pump or a buildup of debris like sludge that is physically blocking the flow of water. When the boiler detects that the outgoing water is significantly hotter than the returning water, it shuts down to prevent the unit from overheating.
This issue is more frequent in autumn when the heating is first switched on after summer, as the pump can seize up after long periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barRestart the boiler using the reset button
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A boiler cover plan would cover faults like F.23 — no call-out fees, no surprises.
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