The boiler has lost the signal from the thermostat sensor in one room-heating zone, so that zone is running on a less precise backup setting until the sensor connection is restored or the sensor is replaced.

DIY fix possible

Turn the boiler off at its on/off switch and at the mains supply, wait about 5 minutes, then turn it back on to clear the fault.Locate the room thermostat or room sensor for heating circuit 2, usually on an inside wall, and check its cable is firmly connected at the sensor end and not cut, pinched, or loose.If the fault returns, contact a Gas Safe registered engineer to check the sensor cable connection at the boiler and test or replace the room sensor.