Your boiler has lost the signal from the room sensor for its second heating zone, so that zone is now being controlled by outside temperature alone and may not heat to the exact room temperature you set.

DIY fix possible

Check the batteries in the wireless room thermostat or sensor for heating circuit 2 and replace them if they are low or flat.Switch the boiler off at its power supply or fused spur for 30 seconds, switch it back on, and check whether the code clears.Make sure the room sensor is not behind furniture, in direct sunlight, or near a draught, and look for any visibly damaged cable. Do not open the boiler casing. If the code returns, book a Gas Safe registered engineer.