Your boiler has lost its connection to the indoor temperature sensor for your main heating area, meaning it cannot accurately tell how warm your rooms are.

Short circuit, room temperature sensor, heating circuit 1 (without mixer)

What causes this fault?

This fault occurs when the electrical signal from your indoor temperature controller is interrupted or becomes inconsistent. The most common reason is a loose connection in the wiring or a failure of the internal sensor component that detects the heat level in your home.