Your boiler has detected an electrical fault with the internal sensor that monitors the temperature of the water leaving the unit, causing it to shut down for safety.

Short circuit: Flow sensor - Flow temperature sensor defective or has short-circuited

What causes this fault?

This fault occurs when the electrical connection to the flow temperature sensor fails, usually due to an internal short circuit or water damage to the wiring. It essentially means the boiler can no longer accurately measure the temperature of the water it is heating, so it shuts down as a protective measure to prevent overheating. The most common reason is simply a component failure within the sensor itself or moisture causing the wires to touch.