Your boiler has stopped working because a sensor that measures the temperature of the water returning to the unit has developed a fault or disconnected.

Return NTC fault

What causes this fault?

This fault occurs when the internal control board can no longer read the temperature signal from the pipe that carries water back into the boiler. The most common reason is that the sensor has physically failed due to age or internal wear, but it can also be caused by a loose connection or damaged wiring. Because the boiler cannot accurately track the water temperature, it shuts down as a safety precaution.