The boiler has lost the signal from its outside weather sensor, so it can't adjust heating to the weather and is running at a fixed setting instead, which is usually not an emergency but should be checked.

DIY fix possible

Switch off the boiler at its on/off switch and at the fused spur, then wait five minutes.Find the outside temperature sensor, usually a small white or grey box on a north or shaded outside wall, and check its cable for visible damage, chafing, or a cut where it enters the wall.If the sensor cable is accessible at the sensor end, check that its plug or connector is fully pushed in and not corroded. Do not open the boiler casing or touch any gas components.Switch the boiler back on and check whether the code clears. If it returns, leave the boiler running at its fixed setting and book a Viessmann-approved engineer to test the sensor and replace it or its lead.