BoilerFaultCodesTechnical 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.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.