BoilerFaultCodesYour boiler has detected that the water is heating up too quickly or isn't circulating properly, causing it to shut down to prevent overheating.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Actual temperature differential between flow and safety temperature sensor is too high
What causes this fault?
This fault occurs when water cannot move through the boiler fast enough to carry heat away, causing a rapid and dangerous rise in temperature. The most common reasons are a failed internal pump that has stopped spinning or a blockage caused by a buildup of sludge and debris within the main heat exchanger. In some cases, it may simply be caused by air trapped in the system or sensors that are providing incorrect temperature readings to the control board.
This fault often appears when the heating is first turned on after summer, as the internal pump can seize up during long periods of inactivity.
DIY fix possible
Check that your radiator valves and any isolation valves are fully openEnsure your system pressure is topped up to between 1 and 1.5 barBleed your radiators to remove any trapped air that might be blocking circulation
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 D4271 — no call-out fees, no surprises.
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Worcester Bosch GB162 is discontinued — time for a new one?
Parts for discontinued models can be hard to find and expensive. A new A-rated boiler could save you money on both repairs and energy bills.
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