Your boiler has lost the signal from its room temperature sensor, so it can't read the room directly, but it should still keep your home heated using its backup settings.

DIY fix possible

Check the room thermostat or room sensor on the wall. If it runs on batteries, replace them with fresh ones and check it is set to the correct heating circuit.Make sure the room sensor is not hidden behind furniture, curtains, or near a radiator or direct sunlight, and that it is mounted securely on the wall.Switch the boiler off at its on/off switch, wait 30 seconds, then switch it back on and check whether the code clears.If the code returns, leave the boiler casing closed and contact a Viessmann-registered heating engineer, as the wiring behind the room sensor and inside the boiler needs a qualified check.