Your boiler has detected a fault with the outdoor temperature sensor, but it is safely using an internal backup to keep your heating running in the meantime.

Outside temp. sensor faulty - heating back-up operation active: Tip: Check the weather compensation is applicable and the menu has not been accidentally switched on in the menu. 1. Check the positioning of the outside weather sensor. 2. Check the weather sensor connections on the control board. 3. Check the resistance of the sensor and the continuity down the cable and replace if required.