Your boiler has stopped working because it cannot communicate with the sensor that monitors the temperature of the water returning to the unit.

Interruption return temperature sensor (NTC): NTC faulty, NTC cable faulty, faulty plug connection on NTC, faulty plug connection on the electronics

What causes this fault?

This fault occurs when the boiler's control board loses its electrical connection with the sensor that monitors water coming back from your radiators. The most common reason is a component failure within the sensor itself or a loose wiring connection that prevents the boiler from safely monitoring heat levels. Because the boiler cannot verify the return temperature, it shuts down as a safety precaution to prevent overheating.