The boiler has lost the signal from the room thermostat sensor for one heating zone, so it is heating that zone based on outside temperature instead of the actual room temperature, which may make it less precise but usually still keeps the house warm.

DIY fix possible

Turn off the boiler at its on/off switch, wait 2 minutes, then turn it back on to clear the fault and see if it returns.Find the room sensor or room thermostat for heating circuit 3 (often in a hallway or living room) and check the cable at the wall and at the sensor for visible damage, pinched wires, or water staining.If the sensor is a wireless or battery unit, replace the batteries, then restart the boiler.If the fault returns or the cable looks damaged, leave it alone and book a Gas Safe registered engineer, as the cable or sensor will need replacing and the boiler's control box should not be opened by a homeowner.