BoilerFaultCodesYour boiler has detected that water is leaving the unit much hotter than it is returning, which usually means the water isn't circulating through your radiators fast enough.
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 the supply sensor and return sensor too large.
What causes this fault?
This fault occurs when the heat generated by the burner isn't being carried away to your radiators quickly enough, causing heat to build up inside the unit. The most common reason is a failing pump that can no longer push the water around your home, though a blockage caused by sludge or debris in the system can also restrict this flow. If the water stays inside the boiler for too long, it overheats and the system shuts down for safety.
This fault often appears during the first cold spell of autumn when the heating is turned on after being dormant all summer, as pumps often seize up during long periods of inactivity.
DIY fix possible
Check that all radiator valves are fully openEnsure the boiler pressure gauge is between 1 and 1.5 barBleed all radiators to remove trapped air that might be blocking water flow
Find a Gas Safe engineer
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A boiler cover plan would cover faults like 2U — no call-out fees, no surprises.
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