Your boiler has shut down because it detected that the exhaust gases leaving the system have reached a dangerously high temperature.

Fault indication / Lockout reset button

What causes this fault?

This fault occurs when the internal sensors detect that the exhaust gases are getting far too hot, which can damage the boiler's internal components. The most common reason is a build-up of debris or limescale inside the main 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 physical blockage in the flue pipe or a failure of the safety thermostat itself.