Your boiler is simply performing a routine safety check to clear out any residual air before it attempts to light.

Pre-purge: before start-up, the boiler is purged for 3 seconds.

What causes this fault?

This fault occurs when the water inside the heat exchanger becomes too hot too quickly, causing the safety sensors to shut the system down to prevent damage. The most common reason is a lack of water circulation, often caused by a failed pump, a blockage in the pipework, or air trapped within the unit. It can also happen if there is not enough water in the system to carry the heat away from the burner.