A: Both. The boiler cannot interpret any demand signals from the user interface, so it will not fire for central heating or domestic hot water.
Before diving into causes, verify you have the correct fault. When D6 appears, you may also notice: viessmann d6 error
Tools needed: Torx T20, fine sandpaper or steel wool, multimeter with microamps. A: Both
Here are some common causes of the D6 error code: When D6 appears, you may also notice: Tools
If the main logic board has a fault (e.g., blown capacitor, damaged processor), it cannot send or receive bus signals. This is a more expensive repair.
If the pressure is fine and a reset doesn't work, the issue is likely internal. A Viessmann Trained Installer or Gas Safe engineer will need to: Installation Instructions - Flow Sensor - Viessmann US