Your boiler has lost the ability to detect the flow of water, so it has shut down to prevent damage from overheating.

Open circuit on flow sensor

What causes this fault?

This fault occurs when the boiler's internal computer can no longer detect a signal from the sensor that monitors the temperature of the water leaving the heat exchanger. It is usually caused by the sensor failing internally or a loose connection in the wiring that links the sensor to the main control board.