BoilerFaultCodesYour boiler has switched itself off because the water temperature inside it is rising much faster than the water temperature being sent to your radiators, usually due to a circulation blockage.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Safety switch off temperature difference too great
What causes this fault?
This fault occurs when the internal water temperature rises too quickly because the heated water cannot leave the boiler fast enough to reach your radiators. The most common reason is a pump failure or a significant blockage in the system, such as a build-up of sludge or debris, which prevents the water from circulating properly.
This fault is most common when you first turn the heating on in autumn, as internal pump parts often seize up after being inactive throughout the summer months.
DIY fix possible
Check that all radiator valves (TRVs) are turned to the highest settingEnsure the boiler pressure is between 1.0 and 1.5 bar on the digital displayCheck if any valves underneath the boiler have been accidentally closed
Find a Gas Safe engineer
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Gas Safe Register →Protect yourself from future repair bills
A boiler cover plan would cover faults like F.23 — 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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