Your boiler started to heat up but the flame went out unexpectedly while it was running.

Flame failure (loss of flame during operation)

What causes this fault?

This fault occurs when the boiler's control system loses the signal that a flame is present while it is already firing. The most common reason is that the flame sensing electrode has become worn or dirty and can no longer detect the heat, although it can also be caused by an unstable gas supply or a failing component preventing the gas from flowing correctly.