Your boiler has detected that the gas valve is not closing properly or there is a communication error between the gas valve and the control board.

Defective gas–valve closure mechanism | Cause: Leak in gas valve assembly, electronic fault

What causes this fault?

This fault occurs when the boiler detects that gas is still flowing or the valve hasn't shut properly after the flame should have gone out. The most common reason is a physical internal leak within the gas valve assembly or a communication failure where the main control board incorrectly reads the valve's position.