Your boiler has detected a failure with the internal gas control valve, meaning it cannot safely ignite to provide heating or hot water.

Gas Valve – Natural Gas Dungs BM771 SL 040

What causes this fault?

This fault occurs when the boiler's main control board loses electrical communication with the gas valve or detects that the valve is physically unable to open. The most common reason is an internal mechanical failure within the valve itself or a loose connection in the wiring harness that prevents the flow of gas required for ignition.