A: That is a classic sign of a frozen condensate pipe. The water produced during combustion freezes at the pipe's exit, causing a backup.
There is nothing worse than waking up to a cold house and a flashing error code on your boiler display. For owners of Viessmann condensing boilers (particularly the Vitodens 100, 200, and 050-W series), encountering the is a common yet distressing event.
The D6 error appears most commonly on:
Water damage near the control board is a common killer. Check inside the boiler casing for:
Prices include parts and labour by an accredited Viessmann technician. Warranty may cover D6 if the boiler is under 5 years old.
Because the D6 error involves the gas valve and combustion components, it is not a DIY fix. A Gas Safe registered engineer (or a licensed technician in your region) will need to open the unit to inspect the internals.