Your boiler has stopped working because a sensor that monitors the temperature of the water returning from your radiators has developed a fault.

Central heating return NTC fault

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the water returning to the boiler begins sending incorrect data or stops communicating entirely. The most common reason is that the internal components of the sensor have degraded over time or the electrical connection has become loose or damaged. Because the boiler can no longer accurately monitor the system temperature, it shuts down as a safety precaution to prevent overheating.