BoilerFaultCodesVaillant d.31 Fault Code
Also written as D31 — same fault.
The Vaillant d.31 fault code means: this code is simply an information display showing whether your boiler is set to fill itself automatically, semi-automatically, or requires you to do it manually.. It appears on 20 Vaillant models in our database.
Technical description: Automatic filling device: 0=Manual, 1=Semi-automatic, 2=Automatic
DIY fix possible
Navigate to the boiler's settings menuLocate the d.31 diagnostic menuAdjust the value to match your specific installation requirements, typically 1 if you have a built-in filling loop
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the d.31 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.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 27 April 2026