BoilerFaultCodesYour boiler has sensed that water is not circulating through the system properly, causing a large temperature gap between the pipes leaving and returning to the unit.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Temperature difference between flow/return NTC too large
What causes this fault?
This fault occurs when the water inside the boiler heats up too quickly because it isn't moving away into your radiators fast enough. The most common reasons are a failed internal pump that has stopped spinning or a blockage in the pipework that is restricting the flow of water. Either way, the boiler shuts down as a safety precaution to prevent the heat exchanger from overheating.
This fault is more common in autumn when heating systems are turned back on after a long summer break, as pumps can seize during periods of inactivity.
DIY fix possible
Ensure all radiator valves and lockshields are fully open.Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge.Bleed all radiators to remove any trapped air pockets that may be blocking flow.
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A boiler cover plan would cover faults like F23 — no call-out fees, no surprises.
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