Your boiler has lost the room temperature reading for one of your heating zones, so it is heating that zone using outside weather readings instead, and it should still keep your home warm.

DIY fix possible

Check that the room temperature sensor for the second heating circuit (usually a small wall-mounted unit near a thermostat) is plugged in securely and has no visible damage to its cable.Make sure the sensor is not blocked by furniture, curtains, or a radiator, and is not in direct sunlight or a draught.Reset the boiler using the reset button or the menu on the control panel, wait around 10 minutes, and check whether the code clears. If it returns, contact a Gas Safe registered engineer to test or replace the sensor.