Your boiler is still detecting a flame even though it has tried to turn the gas supply off, which is a safety concern that requires an engineer.

Ionisation detected after burner stop

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect a flame even after the gas supply has been told to shut down. The most common reason is a gas valve that hasn't closed fully or a sensor that is incorrectly reporting a flame due to an electrical fault. Because this involves gas potentially flowing when it shouldn't, the boiler shuts down immediately as a safety precaution.