Your boiler is detecting a flame at the wrong time, suggesting that gas might be leaking into the burner when it should be shut off.

Flame detected during ignition cycle. Check gas valve closes fully. Replace. Check ionisation probe & condensate outlet

What causes this fault?

This fault occurs when the boiler's sensors detect a flame while the gas valve should be closed, or it senses a 'ghost flame' due to an electrical error. The most common reason is a faulty gas valve that is failing to shut off properly, causing gas to linger in the burner. It can also be triggered by a contaminated sensor probe that is sending an incorrect electrical signal to the main control board.