Your boiler has stopped working because it has detected a fault with the internal sensor that monitors the temperature of the water leaving the unit.

Temp.sensor fault: Short-circuit in flow temperature sensor.

What causes this fault?

This fault occurs when the boiler's control board receives an incorrect electrical signal from the flow sensor, usually caused by an internal failure of the component itself. The most common reason is that the sensor has suffered an electrical short-circuit, which prevents the boiler from safely monitoring how hot the water is before it leaves the unit.