Your boiler has detected that the exhaust temperature is being measured incorrectly and has shut itself down as a safety precaution.

Flue sensor out of range. Flue sensor measures a temperature outside normal working range.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring exhaust temperature signals a reading that is physically impossible or dangerously high. The most common reason is a failing sensor sending incorrect data, but it can also be triggered by a build-up of debris or scale within the heat exchanger causing the system to overheat.