Your boiler has stopped working because it cannot communicate with the sensor that monitors the temperature of the water returning from your radiators.

Interruption: Return sensor - Return temperature sensor defective or not connected

What causes this fault?

This fault occurs when the boiler's main control board loses its electrical connection to the sensor that monitors water coming back from your radiators. The most common reason is that the sensor itself has developed an internal electrical fault, or the thin wires connecting it to the system have become loose or damaged over time.