Your boiler is displaying the current outdoor temperature because an external weather sensor is connected, which is a normal status reading rather than a breakdown.

Outside temperature: Current outside temperature (with outside temperature sensor connected)

What causes this fault?

This code appears because your heating system has an optional weather compensation sensor installed on the outside of your property. It acts as an information display rather than a mechanical failure, simply showing the boiler's reading of the current air temperature. This allows the boiler to automatically adjust its efficiency based on how cold it is outside.