Your boiler is lighting up but the flame goes out straight away, preventing the system from producing heat.

The ionisation (the flame) directly extinguishes after ignition.

What causes this fault?

This fault occurs when the boiler successfully creates a flame, but the safety sensors fail to detect it, causing the system to shut down for safety. The most common reason is a dirty or worn-out sensing probe that can no longer 'see' the fire, or an issue with the gas valve not providing a steady enough flow to maintain the flame.