BoilerFaultCodesYour boiler has detected that the water is heating up too quickly or isn't moving through the system properly, causing it to overheat and shut down for safety.
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 happens when heat cannot escape the boiler quickly enough, causing the internal temperature to spike dangerously fast. It is usually caused by a pump failure that stops water from moving or a blockage in the pipes, such as a buildup of sludge or closed valves, that prevents circulation. Effectively, the boiler is producing heat but has nowhere to send it, so it shuts down to prevent damage.
This issue often appears when the heating is turned back on for autumn, as pumps can seize up after being idle throughout the summer.
DIY fix possible
Check that at least two or three radiator valves are fully open to allow water flow.Ensure the large blue and red isolation valves underneath the boiler are in the 'open' position.Try resetting the boiler by turning the control knob to 'Reset' and then back to the original setting.
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A boiler cover plan would cover faults like FU — no call-out fees, no surprises.
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Could it be time for a new boiler?
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