Your boiler has detected an electrical fault with the sensor that monitors water flow, meaning it cannot safely determine if water is moving through the system.

Flow sensor short circuit. Check cable/connections not damaged. Renew sensor/cable. Check connections on control

What causes this fault?

This fault occurs when the electrical signal from the water flow sensor is disrupted, usually due to a physical break or a short circuit in the wiring. The most common reason is moisture ingress damaging the sensor's internal electronics or vibrating wires rubbing against internal components until the insulation wears through. When this happens, the boiler shuts down because it can no longer verify if water is safely moving through the heat exchanger.