Your boiler has lost connection with the temperature sensor that controls your second heating zone, meaning it cannot properly regulate the temperature for that part of your home.

Heating circuit M2 room temperature sensor lead break

What causes this fault?

This fault occurs when the boiler's control system can no longer detect a signal from the temperature sensor assigned to your second heating zone. It is usually caused by a loose electrical connection, a break in the thin sensor wiring, or the internal component of the sensor failing due to age.