Your heating system is having trouble measuring the temperature for your second heating area, which often means that specific zone won't warm up correctly.

Central Heating Zone 2 Thermistor Fault.

What causes this fault?

This fault occurs when the boiler loses communication with the temperature sensor dedicated to your second heating zone, such as an upstairs circuit or underfloor heating area. The most common reason is either a physical failure of the internal sensor component or a loose connection in the wiring that prevents the boiler from monitoring how hot the water is in that specific part of your home.