Your boiler has detected that the exhaust gases are not being monitored correctly and has shut down for safety.

Failed flue temperature – the flue sensor needs checking or replacing

What causes this fault?

This fault occurs when the boiler's internal safety system can no longer accurately measure the temperature of the exhaust gases leaving the flue. The most common reason is that the sensor itself has degraded or developed a short circuit, causing it to send an incorrect signal to the main control board. If the boiler cannot verify that these gases are within a safe temperature range, it will shut down to prevent potential damage to the internal components.