Your boiler has detected that it is getting too hot inside, often because something is blocking the flow of heat or a sensor is faulty.

Flue sensor fault or burner output modulates to maintain temperature set (potential heat exchanger obstruction)

What causes this fault?

This fault occurs when the boiler's internal components reach a temperature that is dangerously high, triggering a safety shutdown to prevent damage. The most common reason is a buildup of limescale or debris inside the heat exchanger, which restricts water flow and prevents heat from being transferred away from the burner effectively.