Your boiler's control unit has lost contact with the thermostat for one of your heating zones, so that zone may not be heating properly, but your boiler itself is likely still working and this is usually a fixable connection or battery problem.

DIY fix possible

Check the batteries in the room thermostat (Vitotrol remote control) and replace them if they are low or flat.Turn the boiler off at its on/off switch, wait 30 seconds, then switch it back on to clear the error and see if it reappears.Check that the cable between the thermostat and the boiler's control unit is not loose or damaged, and that the thermostat is fitted in its base or mounted securely.If the code returns, contact a Gas Safe registered engineer to check the wiring, the mixer extension module and the thermostat.