Your boiler is having trouble communicating with the control box that manages your second heating zone, meaning that specific area may not heat up correctly.

Mixer M2 continues to regulate: Communication fault - extension kit for mixer circuit M2

What causes this fault?

This fault occurs when the boiler's main brain loses its connection with the electronic module that controls your underfloor heating or second radiator zone. It is usually caused by a loose wiring connection or a component failure within the external control box that manages that specific heating circuit.