Your boiler has sensed that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent internal damage.

FLUE GAS OVERHEAT

What causes this fault?

This fault occurs when the internal temperature of the exhaust fumes exceeds a safe limit, triggering an emergency shutdown to protect the boiler's components. The most common reason is a build-up of limescale or debris inside the heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be caused by a faulty sensor giving an incorrect reading or a blockage in the system's circulation.