BoilerFaultCodesVaillant d.66 Fault Code
Also written as D66 — same fault.
The Vaillant d.66 fault code means: your boiler is simply indicating that the 'warm start' feature is currently active, which keeps a small amount of water pre-heated so your hot water arrives at the tap more quickly.. It appears on 20 Vaillant models in our database.
Technical description: Activation of the warm start function for hot water
DIY fix possible
This is a status message, not a fault code.If you wish to save energy, access the boiler menu.Locate the 'Warmstart' setting and switch it to 'Off'.
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Gas Safe Register →Vaillant models that show the d.66 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT pure
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 835
- ecoTEC plus
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 65
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus System
- ecoTEC sustain
- Home System
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 27 April 2026