Your boiler has stopped working because it cannot get a temperature reading from the main pipe where hot water leaves the unit.

Outlet probe with contact open

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor responsible for monitoring the temperature of the water leaving the unit. The most common reason is that the sensor has developed an internal electrical failure or the wiring connecting it to the main circuit board has become loose or damaged. Without this data, the boiler shuts down as a safety precaution to prevent the system from overheating.