Your boiler has detected that gas is potentially still flowing or a flame signal is present when it should be turned off, which is a safety concern.

Gas valves do not close; check the voltage on the gas valve. If a voltage is measured on both the gas valves (> 20 V), the control panel should be replaced. When ionisation occurs while the unit is not operational, the ionisation circuit should be checked. If the control panel and the ionisation function properly, the gas valve has to be replaced.

What causes this fault?

This fault occurs when the boiler's internal sensors detect a flame or gas flow even after the system has been told to shut down. The most common reasons are a gas valve that is physically stuck open or a control board that is incorrectly sending power to the valve when it shouldn't. It can also be caused by a faulty sensor that is 'seeing' a flame that isn't actually there.