Your boiler has failed to keep its flame lit while trying to automatically adjust its settings for efficient burning.

What causes this fault?

This fault occurs when the boiler's internal computer tries to adjust the gas and air mixture to ensure it is burning fuel as efficiently as possible, but the flame goes out during the process. The most common reason is that the sensor responsible for 'seeing' the flame is dirty or worn out, or the gas valve isn't providing a steady enough flow for the boiler to complete its self-check. Because the boiler cannot guarantee a safe and efficient burn, it shuts down for safety.