The of your Viessmann boiler (e.g., Vitodens 100-W, 200-W) If the error appeared after heavy rain or recent DIY work Whether you have heat and hot water right now
If water squirts out of the vessel's air valve during testing, the internal diaphragm has split, and the entire expansion vessel must be replaced.
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Safety first
If the manual gauge shows normal pressure but the boiler reads a D6 error, the engineer will test the pressure switch using a multimeter and replace the sensor if it is found to be defective.
Are you seeing this error on a gas boiler or a Vitocal heat pump?
If the boiler fires up and the error disappears, it may have been a temporary glitch. 2. Check the Flue Terminal viessmann d6 error
In simple terms: The "brain" of your boiler isn't talking properly to the "screen."
While some initial checks can be performed by a homeowner, most D6 repairs require a Gas Safe registered engineer due to the involvement of the gas burner and internal electrical components. Viessmann Fault Code Checker
The filling loop connects your mains water supply to the heating system. If the filling loop valves are not completely closed, or if the valve seals have degraded, mains water will continuously leak into your heating system, causing the pressure gauge to climb continuously. 3. Expansion Vessel Failure The of your Viessmann boiler (e
If the error appears only occasionally (e.g., once a week), it points to a loose connection or failing power supply. Fixing it early prevents a complete control board failure—which can cost $500+ in parts alone.
If you hear the fan spin up immediately before the boiler attempts to ignite, the fan relay might be stuck.