d8 Medium

Viessmann d8 Fault Code

Also written as D8 — same fault.

The Viessmann d8 fault code means: your boiler has detected an external signal or safety switch connected to it that is currently reporting an error, meaning the system has paused to prevent operation until the external issue is resolved.. It appears on 2 Viessmann models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Input DE3 at extension EA1 reports a fault
Typical repair cost £120-250 (including labour)

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 d8 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Viessmann d8 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