D.066 Low

Vaillant D.066 Fault Code

Also written as D066 — same fault.

The Vaillant D.066 fault code means: your boiler is signalling that the 'warm start' feature, which keeps a small amount of water pre-heated for faster hot water taps, is currently active or adjusting.. It appears on 3 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Warmstartactivation

DIY fix possible

Navigate to the boiler's menu settingsLocate the 'Warmstart' or 'Comfort' mode optionToggle the setting to 'Off' if you do not require instant hot water, or 'On' if you wish to use the feature

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Vaillant models that show the D.066 fault code

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

Source

Vaillant D.066 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