Your boiler has stopped working because it cannot accurately measure the temperature of the water returning to the unit.

Return temperature sensor interruption

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor has developed an internal electrical fault or the connecting wires have become loose or damaged. Because the boiler can no longer track these temperatures safely, it shuts down to prevent overheating.