BoilerFaultCodesVaillant d.23 Fault Code
Also written as D23 — same fault.
The Vaillant d.23 fault code means: this code is not actually an error; it is a status message showing that your boiler is currently set to 'summer' mode, meaning it will only provide hot water and not central heating.. It appears on 32 Vaillant models in our database.
Technical description: Summer/winter function (control knob) 1 = winter; 0 = summer
DIY fix possible
Locate the heating control knob on the front of your boiler panelTurn the dial clockwise to the 'Winter' position (indicated by a radiator icon)Observe if the heating system begins to activate
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Vaillant models that show the d.23 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX
- ecoMAX 613 Repair: £120-250
- ecoMAX 618 Repair: £120-250
- ecoMAX 622 Repair: £120-250
- ecoMAX 635 Repair: £120-220
- Ecomax Condensing 600/800 series Repair: £120-180
- ecoMAX System 612 Repair: £100-200
- ecoTEC 65kW
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC plus aquaPLUS
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828 Repair: £120-220
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant d.23 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant Ecomax Condensing 600/800 series manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026