Your boiler has shut down because its internal temperature sensors have detected that the exhaust gases are getting too hot, which could damage the system.

What causes this fault?

This fault occurs when the internal sensors detect that the waste gases leaving your boiler are significantly hotter than they should be, triggering a safety shutdown to prevent internal damage. The most common reason is a buildup of debris or limescale 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 lack of proper water flow through the system.