Your boiler is detecting a flame even though the gas valve should be closed, which is a safety precaution that stops the boiler from running.

Flame On Before Gas Valve On

What causes this fault?

This fault occurs when the boiler's internal sensors detect a flame while the system is in a standby state and the gas valve should be shut. It is often caused by a faulty sensor sending a false signal, a leaking gas valve that is letting fuel through when it shouldn't, or a moisture-damaged control board misinterpreting electrical signals.