Your boiler has detected a fault with the sensor that monitors the water returning to the unit, causing the system to stop working for safety.

Return probe short-circuited

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the water entering the boiler fails, typically due to an internal electrical short. The most common reason is regular wear and tear on the sensor's internal components, though it can also be triggered by moisture ingress or a damaged wiring connection. Because the boiler can no longer accurately monitor water temperature, it shuts down to prevent the system from overheating.