Your boiler has lost the ability to accurately measure the temperature of the water being sent to your radiators, so it has shut down to prevent overheating.

CH flow NTC sensor defective.

What causes this fault?

This fault occurs when the boiler's internal thermometer, which monitors the water temperature leaving the heat exchanger, fails to provide a correct reading. The most common reason is that the sensor's internal electrical resistance has drifted out of range due to age, or the connection pins have become corroded, preventing the boiler from operating safely.