d.31 Medium

Vaillant d.31 Fault Code

Also written as D31 — same fault.

The Vaillant d.31 fault code means: your boiler has detected an issue with its internal system that automatically tops up the water pressure, meaning it may be unable to maintain the correct levels needed to function properly.. It appears on 8 Vaillant 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: Automaticfillingdevice
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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Vaillant models that show the d.31 fault code

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

Source

Vaillant d.31 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026