BoilerFaultCodesVaillant d.04 Fault Code
Also written as D04 — same fault.
The Vaillant d.04 fault code means: your boiler is displaying the current temperature of the water inside your hot water storage cylinder; this is a status information code rather than a system failure.. It appears on 22 Vaillant models in our database.
Technical description: Water temperature in the cylinder
DIY fix possible
This fault may be fixable without an engineer. Check your manual first.
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Gas Safe Register →Vaillant models that show the d.04 fault code
Click a model for the full fault code list and model-specific repair notes.
- aquaPLUS
- 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 Aquaplus
- ecoTEC plus System
- ecoTEC sustain
- Home System
Source
Vaillant d.04 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