BoilerFaultCodesVaillant D.23 Fault Code
Also written as D23 — same fault.
The Vaillant D.23 fault code means: your boiler is currently set to summer mode, which means it is only heating your tap water and has temporarily turned off the central heating.. It appears on 37 Vaillant models in our database.
Technical description: Summer/Winter function
What causes the Vaillant D.23 fault?
This status occurs when the heating system has been manually or automatically switched to its summer setting to save energy. It physically prevents the boiler from sending heat to your radiators while still allowing it to provide hot water for your taps and showers. The boiler is not actually broken; it is simply following an instruction to prioritise hot water over room heating.
This code is most frequently seen during the spring and summer months when homeowners turn off their heating as the outdoor temperature rises.
DIY fix possible
Locate the radiator temperature dial or button on the boiler control panelTurn the dial clockwise or press the '+' button to increase the target heating temperatureCheck that the display no longer shows 'Summer' or the radiator icon with a cross through it
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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 824E Repair: £120-220
- ecoMAX 828E
- Ecomax Condensing 600/800 series Repair: £120-180
- ecoMAX System 612 Repair: £100-200
- ecoTEC
- ecoTEC 65kW
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC plus aquaPLUS
- ecoTEC Pro / ecoTEC Plus
- 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 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026