Your boiler has detected a fault with the sensor that monitors exhaust gases, and as a safety precaution, it will not operate until this is fixed.

Flue sensor damaged

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of the exhaust gases becomes faulty or disconnected. The most common reason is natural wear and tear or heat damage to the sensor's thermistor, which prevents it from sending an accurate reading to the main control board. As a safety measure, the boiler shuts down to prevent any risk of the flue system overheating.