BoilerFaultCodesViessmann d6 Fault Code
Also written as D6 — same fault.
The Viessmann d6 fault code means: your boiler has lost communication with an external accessory, such as a solar thermal controller or an additional heating control module, which is preventing the system from operating correctly.. It appears on 2 Viessmann models in our database.
Technical description: Input DE1 reports a fault at extension EA1
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Viessmann models that show the d6 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Viessmann d6 fault code information on this page is sourced and cross-checked against the official Viessmann service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026