Your boiler has shut down because a safety device or an external controller has detected an issue and stopped the system from running.

External device fault (UHT/CPA)

What causes this fault?

This fault occurs when an external safety device or a limit thermostat detects that the boiler is getting too hot, triggering an emergency shutdown to prevent damage. It is often caused by a lack of water flow through the heat exchanger or a loose connection in the wiring that links the boiler to its external controls. In some cases, the sensor itself may have failed and is sending an incorrect signal to the main control board.