Your boiler has shut down because the exhaust gases have become too hot, which is a safety measure to prevent damage to the system.

Flue NTC sensor over temperature

What causes this fault?

This fault occurs when the internal temperature sensor detects that the exhaust gases are getting far too hot, triggering an immediate shutdown to protect the boiler's internal plastic components. The most common reason is a buildup of limescale or debris inside the main heat exchanger, which prevents heat from transferring into your water and sends it out through the flue instead. It can also be caused by a lack of water circulation or a faulty sensor giving an incorrect reading.