Your boiler is currently showing the live temperature reading of the sensor responsible for keeping your hot water pre-heated.

Current temperature for warm stat sensor

What causes this fault?

This isn't actually a fault, but a live data reading showing the temperature of the internal sensor that manages your pre-heated hot water. It appears when the boiler is actively monitoring the heat of the water held inside the heat exchanger to ensure you get hot water quickly when you turn on a tap. If you see this, it simply means the 'Warmstart' function is active and the system is checking the current heat level.