The boiler has lost the signal from the room thermostat for its third heating zone, so it is heating that zone based on outside temperature instead, and it should still keep your home warm.

DIY fix possible

Turn the boiler off at its on/off switch, wait 2 minutes, then switch it back on and check whether the code clears.Check the room thermostat or room sensor for heating circuit 3 is switched on, and replace its batteries if it runs on them.Look at the wiring or plug connection between the room sensor and the wall, and make sure it is secure, without damage, and not loose. Do not open the boiler casing.