BoilerFaultCodesYour boiler has detected that water is getting too hot too quickly because it isn't circulating through your radiators or pipes correctly.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: DIFF GREATER THAN 50°C CHECK ISOLATION VALVES ARE OPEN CHECK PUMP CHECK RAD VALVES OPEN CHECK SYSTEM BLOCKAGES
What causes this fault?
This fault occurs when the temperature difference between the water leaving the boiler and returning to it becomes dangerously high, usually because the water isn't moving through the system. The most common reasons are a failed internal pump that can't push the water around or a blockage in the system, such as closed radiator valves or a build-up of sludge. This causes the heat to stay trapped inside the boiler, triggering an emergency shutdown to prevent internal damage.
This fault is frequently seen at the start of autumn when heating systems are turned back on after a long summer break, as internal pump parts often seize up during periods of inactivity.
DIY fix possible
Check that all radiator valves (TRVs) are turned to a high settingEnsure the blue and white isolation valves underneath the boiler are in the open positionGently bleed your radiators to remove any trapped air pockets
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A boiler cover plan would cover faults like FU — no call-out fees, no surprises.
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