Your boiler has lost its connection to one of the temperature sensors used to control your heating, though it will likely keep running in a basic backup mode.

Control mode without room influence - Heating circuit M2 room temperature sensor shorted out

What causes this fault?

This fault occurs when the boiler loses a reliable signal from the sensor that monitors the temperature inside your living area. It is typically caused by a frayed or loose connection in the wiring, or a internal failure within the sensor itself, meaning the boiler no longer knows how warm your rooms actually are. Instead of shutting down completely, the system reverts to a basic setting to ensure you still have some heat.