Your boiler has detected a loose connection or a fault with the sensor that monitors the temperature of the water returning to the unit.

Short circuit - flow-NTC (<130°C) | NTC-plug shorted to casing, NTC defective

What causes this fault?

This fault occurs when the boiler's control board receives an impossible electrical reading from the sensor that monitors cooled water returning from your radiators. The most common reason is that the sensor has developed an internal electrical short or the wiring connecting it to the main board has become loose or damaged. Because the boiler can no longer safely track the water temperature, it shuts down to prevent overheating.