Your boiler is struggling to detect the flow of water, which means it cannot tell if the taps are running or if it needs to heat your home.

Water flow selection with water flow meter input

What causes this fault?

This fault occurs when the boiler's internal sensors detect that water is heating up too quickly or not flowing away from the heat exchanger fast enough. The most common reasons are a failed or blocked circulation pump, air trapped in the system, or a buildup of sludge and debris that has restricted the internal pipework. Because the heat cannot be moved into your radiators effectively, the boiler shuts down to prevent the unit from overheating.