Your boiler is failing to stay lit after it tries to start up, meaning it cannot provide you with heating or hot water.

No flame after the ignition phase

What causes this fault?

This fault occurs when the boiler successfully creates a spark and ignites the gas, but then fails to detect that the flame is actually burning. The most common reasons are a dirty or worn-out detection sensor that can't 'see' the fire, or a faulty gas valve that isn't providing a steady enough flow of fuel to keep the flame stable.