BoilerFaultCodesWhat the manual says
Manufacturer manualWater pressure > 2,9 bar
What causes this fault?
This fault occurs when the internal water pressure exceeds a safe limit, often because the filling loop used to top up the system has been left slightly open or is leaking. It can also happen if the expansion vessel, which manages heat-related pressure changes, has failed or lost its air charge. In some cases, a faulty heat exchanger can allow fresh mains water to leak into the heating circuit, causing the pressure to rise continuously.
This issue often surfaces when the heating is first turned on for winter, as the expansion of warming water puts extra stress on a system that might already be overfilled or have a failing expansion vessel.
DIY fix possible
Check that the internal or external filling loop valves are fully closed.Locate a radiator bleed valve and use a key to drain small amounts of water into a container.Monitor the pressure gauge until it reaches between 1.0 and 1.5 bar.If the pressure continues to rise on its own, call a Gas Safe engineer to check the expansion vessel.
Avoid the next call-out bill
A cover plan would handle faults like S.41: no call-out fees, no surprise bills.
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