The boiler has lost contact with the wireless room thermostat that controls the heating, so it may not be heating your home the way it normally does, but this is usually a simple communication fault and not a safety danger.

DIY fix possible

Check the batteries in the room thermostat (Vitotrol remote control) and replace them if they are low or flat.Make sure the thermostat is within range of the boiler's receiver and that nothing metal or a new electrical device sits between them.Turn the boiler off at its on/off switch, wait about 30 seconds, then turn it back on and see if the code clears.If the fault remains, check the thermostat's connection cable at the thermostat and at the boiler's control terminals, but do not open the boiler casing. Contact a Viessmann-registered engineer if the code returns.