Your boiler is failing to start because it cannot confirm that the internal fan is effectively clearing exhaust gases, which prevents it from igniting safely.

The Air pressure switch has not closed.

What causes this fault?

This fault occurs when the boiler's internal drainage system becomes backed up with liquid that can no longer escape. The most common reason is a blockage in the external pipe, often caused by a buildup of sludge or debris within the condensate trap itself. When the water level rises too high inside the unit, a safety sensor shuts the system down to prevent internal leaking or damage.