Your boiler is detecting a flame remaining inside the unit for a few seconds after it should have been shut off.

The flame is extinguished too late after the gas valve closes

What causes this fault?

This fault occurs when the boiler's sensors detect a flame for several seconds after the heating cycle has finished and the gas should have stopped flowing. The most common reason is a gas valve that is physically sticking open or an electrical fault in the control board that fails to cut power to the valve. In some cases, a carbon-coated sensing electrode may also be providing a false signal back to the boiler.