Your boiler thinks it sees a flame even though it isn't currently firing, which is a safety conflict that prevents it from starting up.

False flame

What causes this fault?

This fault occurs because the internal sensors detect a flame after the boiler has finished its heating cycle or before it has even started. The most common cause is a faulty ionisation electrode that is sending an incorrect signal to the control board, or a gas valve that is failing to close fully and allowing a tiny amount of fuel to stay lit.