Your boiler has detected that a temperature sensor responsible for monitoring water returning to the unit has disconnected or broken, preventing it from running safely.

Return sensor open circuit. Check cable/connections not damaged. Renew sensor/cable. Check connections on control

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors water temperature returning from your radiators. This is typically caused by the internal sensor failing due to age or the electrical wiring connecting it to the main board becoming loose or corroded. Because the boiler can no longer monitor these temperatures to prevent overheating, it shuts down as a safety precaution.