Your boiler has lost connection with the sensor that monitors water returning from your radiators, meaning it cannot safely regulate its temperature.

Return probe with contact open

What causes this fault?

This fault occurs when the boiler's control board stops receiving a signal from the sensor that monitors the temperature of the water coming back from your heating system. The most common cause is a failed internal sensor or a loose wire connection, which prevents the boiler from knowing if the system is overheating.