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

Temp.sensor fault: Return temperature sensor defective or not connected.

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors water coming back from your radiators. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be triggered by a loose or damaged connection in the internal wiring.