Your boiler has detected a problem with the sensor that monitors exhaust gases, and it has shut down for safety purposes.

Flue sensor fault

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the exhaust gases signals that it is failing or providing an unrealistic reading. The most common reason is that the sensor has developed an electrical fault due to age, or the internal wiring has become damaged over time. Because this sensor is a critical safety component that prevents the boiler from overheating, the system will shut down immediately to prevent damage.