Your boiler has shut down because a sensor has detected that the exhaust gases are getting too hot or the sensor itself has failed.

What causes this fault?

This fault occurs when the sensor monitoring your boiler's exhaust gases detects temperatures that are high enough to potentially damage the plastic internal components. The most common reasons are a buildup of debris inside the main heat exchanger restricting heat transfer or a simple electrical failure of the sensor itself. In some cases, it can also be triggered if the boiler is over-firing or if the burner has not been serviced and cleaned recently.