d.66 Low

Vaillant d.66 Fault Code

Also written as D66 — same fault.

The Vaillant d.66 fault code means: your boiler is simply letting you know that the 'warm start' feature is currently active, which keeps a small amount of water pre-heated so you get hot water faster when you turn the tap on.. It appears on 9 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: Activation of the warm start function for hot water

DIY fix possible

This is a status message, not a fault, so no repair is required.If you wish to turn this feature off to save energy, navigate to the hot water settings on your boiler display.Locate the 'Warmstart' or 'Comfort' mode setting and select 'Off'.

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Vaillant models that show the d.66 fault code

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

Source

Vaillant d.66 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