BoilerFaultCodesYour boiler has detected that heat is not moving away from the unit quickly enough, often because water flow is being blocked or restricted.
Book a Gas Safe engineer within 24–48 hours. Your boiler may be unsafe or could break down completely if left.
Technical description: Difference greater than 50c – check isolation valves are open
What causes this fault?
This fault occurs when there is a massive temperature gap between the water entering and leaving the boiler, suggesting the heat has nowhere to go. It is usually caused by a physical blockage in the pipework, such as a buildup of sludge, or a mechanical failure in the pump that prevents water from circulating. The boiler shuts down immediately to protect the internal components from overheating and potential heat stress damage.
This issue often surfaces in autumn when the heating is turned on for the first time after summer, as stagnant debris in the pipes can settle and block the system.
DIY fix possible
Check that the blue and red lever valves underneath the boiler are turned fully to the open positionEnsure all radiator valves (TRVs) are turned up to a high settingCheck your central heating filter (if fitted) to see if it is blocked or needs cleaning
Find a Gas Safe engineer
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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?
If repair costs are adding up, a new energy-efficient boiler could be the smarter long-term choice.
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