Your boiler thinks a flame is present even though it hasn't been safely ignited, causing the system to shut down for safety.

False Flame Lockout

What causes this fault?

This fault occurs when the boiler's internal sensor detects a flame signal at a point in the starting sequence where there shouldn't be one. The most common reason is a faulty flame sensing electrode that is sending a wrong signal to the control board, or a gas valve that is leaking a tiny amount of gas into the burner area before it is supposed to open.