Your boiler has shut down because it has detected an unsafe increase in internal temperature, likely caused by a sensor fault or an issue with water flow.

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What causes this fault?

This fault occurs because the boiler's internal sensors have detected that the water temperature has risen too quickly or exceeded a safe limit. It is usually caused by a lack of water flow, often due to a pump failure or a blockage, which prevents the heat from being carried away from the heat exchanger. In some cases, the sensor itself may have become detached or faulty, sending an incorrect reading to the control board.