Your boiler has identified a communication problem with the sensor that monitors water returning from your radiators, preventing the system from operating correctly.

Open circuit on return sensor

What causes this fault?

This fault occurs when the boiler's main control board loses its connection with the sensor that measures the temperature of water returning from the radiators. The most common reason is that the internal components of the sensor have failed, or the electrical wiring connecting it to the brain of the boiler has become loose or damaged.