Your boiler is still detecting a flame even after it should have turned off, which suggests gas may still be flowing into the unit when it shouldn't be.

Ionisation occurs when heat is no longer required, the gas valve does not close.

What causes this fault?

This fault occurs when the boiler's sensors detect heat or a flame after the heating cycle has finished. The most common reason is a mechanical failure in the gas valve that prevents it from closing fully, or a faulty sensor that is incorrectly reporting a flame is still present. Because the boiler believes gas may still be burning when it should be off, it will shut down immediately for your safety.