Your boiler is pausing briefly to check the water flow and fan speed are working correctly before it tries to fire up.

Fan and water pump return water

What causes this fault?

This status occurs when the boiler's internal computer is verifying that the pump is moving enough water and the fan is spinning at the right speed before ignition. It is essentially a safety check to ensure the heat exchanger won't overheat once the flame starts. If the display stays on this code for a long time, it usually means the pump is struggling to move water or a sensor is providing an inaccurate reading.