Your boiler has shut down because it has detected that the exhaust gases are becoming dangerously hot, which usually suggests a serious internal blockage or a lack of water circulation.

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are reaching unsafe temperatures, triggering an emergency shutdown to protect the appliance. The most common reason is a lack of water flowing through the heat exchanger to carry the heat away, often caused by a failed pump or a significant internal blockage. In some cases, it can also be triggered by a heavy buildup of limescale or debris within the main heat exchanger itself.