Viessmann D6 Error -

status using the boiler’s diagnostic menu to see if an external component is sending a "block" signal.

The is a specific fault code that appears primarily on heat pumps like the Vitocal 100 or 111-S and certain high-capacity boilers like the Vitodens 200-W (120/150 kW models). What the d6 Error Means viessmann d6 error