Your boiler has detected a problem with the sensor that monitors the temperature of the exhaust gases, and it has shut down as a safety precaution.

Flue thermistor fault

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring the temperature of the waste gases becomes faulty or loses its connection to the main control board. The most common reason is that the sensor has physically failed due to high heat exposure over time, or the electrical wiring connecting it has become loose or damaged. Because this sensor prevents the boiler from overheating and damaging the flue, the system shuts down immediately for safety.