Your boiler has failed to detect a flame, which means it cannot stay on safely and has stopped working for your protection.

Check the connection of the ionisation pin, use the pin to measure the ionisation current in series (this current should amount to at least 2 µAC).

What causes this fault?

This fault occurs when the boiler's control system cannot confirm that a flame has successfully lit, causing it to shut down for safety. The most common reasons are a worn-out sensor that can no longer 'see' the flame, a build-up of carbon on the probe, or a faulty internal component failing to send the ignition spark.