Ensure all isolation valves under the boiler or near the heat pump are fully open. When to Call a Professional
A: Yes. A shorted outdoor sensor (common after lightning storms) can drop the entire bus voltage. Disconnect it—if D6 disappears, replace the sensor. viessmann d6 error
An external device like a low water cut-off or a flow switch has triggered a shutdown. Ensure all isolation valves under the boiler or
. This is usually detailed in your installer's wiring diagram. Troubleshoot the Peripheral: Disconnect it—if D6 disappears, replace the sensor
A: Some models store D6 as a history code without active lockout. Check your service menu. If boiler works fine, it may have been a one-time glitch.
Press the "Reset" button (usually labeled with a lightning bolt or "R"). Sometimes a temporary glitch causes the error. Inspect the Tubes:
If the D6 error persists after a reset, it likely points to a mechanical failure that requires a Gas Safe registered engineer