Your boiler has detected an internal electrical fault with the part that controls the gas supply, meaning it has safely shut itself down to prevent operation.

Gas valve relay faulty

What causes this fault?

This fault occurs when the boiler detects that the water coming back from your radiators is much hotter than the water it just sent out, which shouldn't happen during normal operation. The most common reason is a lack of water flow caused by a blockage, trapped air, or a failing pump, but it can also be triggered if the electronic sensors that measure temperature become faulty and provide 'garbage' readings to the control board.