Your boiler is having trouble reading the water temperature for one of the specific heating zones in your home, which may cause that part of the system to stop working correctly.

Return sensor defective (zone 3)

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of the water returning from your third heating zone fails or sends an incorrect signal to the main control board. The most common reason is a physical failure of the internal thermistor component or loose wiring connections that prevent the boiler from monitoring the system's heat levels accurately. In some cases, it can be caused by moisture ingress or a buildup of debris affecting the sensor's sensitivity.