Your boiler is struggling to keep its flame lit during operation, which is causing it to stop and restart frequently.

No flame signal. With low output the burner starts automatically after cleaning. If the fault occurs repeatedly, the setting of the CO2 valves need checking

What causes this fault?

This fault occurs when the boiler's internal sensors lose track of the flame while it is firing at a low power level. It is usually caused by a dirty or worn flame sensor that can no longer 'see' the fire, or a gas valve that isn't delivering the precise amount of fuel needed to maintain a stable small flame.