Your boiler is still detecting a flame even after it has told the gas to turn off, which is a safety conflict that causes the system to shut down.

Flame rectification present 4 secs. after gas valve turns off | Check gas valve, burner tubes and injectors or electronics defect

What causes this fault?

This fault occurs when the boiler's internal sensors continue to detect heat or an electrical signal from a flame even after the gas supply should have been cut off. The most common reason is a leaking gas valve that isn't closing fully, or a fault on the main circuit board that is misinterpreting signals from the burners.