Your boiler thinks a flame is still burning inside even after it has been told to turn off, which is a safety conflict.

Flame detected even though appliance is switched off

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame signal even after the gas supply has been shut off. The most common reason is a contaminated flame electrode sending a false signal, or a fault within the main control board that is misinterpreting electrical noise as a live flame.