Your boiler has shut down because the internal water temperature is heating up much faster than it can be circulated away, usually meaning the water is stuck or the pump is struggling.

Water shortage, temperature rise too quick. Pump blocked, insufficient pump output, air in appliance, water pressure too low

What causes this fault?

This fault occurs when the heat inside the boiler builds up too quickly because the heated water isn't moving through your radiators fast enough. The most common reasons are a failing circulation pump, a blockage in the system like sludge or limescale, or air trapped within the pipes.