Your boiler has detected an electrical fault with the sensor that monitors exhaust gases, causing the system to shut down for safety.

Flue sensor short circuit – check connections to flue sensor

What causes this fault?

This fault occurs when the electrical signal from the exhaust gas sensor becomes interrupted, usually because the component has failed internally or the wiring has become damaged. The sensor is designed to monitor the temperature of the fumes leaving your home, and if it detects an electrical short, the boiler will shut down immediately to prevent potential overheating or safety risks.