BoilerFaultCodesVaillant d.40 Fault Code
Also written as D40 — same fault.
The Vaillant d.40 fault code means: this code is simply a status message from your boiler showing you the current temperature of the water flowing through your heating system, rather than an error or a fault.. It appears on 32 Vaillant models in our database.
Technical description: Actual flow temperature
DIY fix possible
Check your display to see if the system is currently heatingObserve if the fault code disappears when the heating or hot water demand stopsMonitor the boiler for any actual error codes appearing instead of the status number
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Vaillant models that show the d.40 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 Repair: £120-250
- ecoFIT Pure 835 Repair: £0-80
- ecoMAX
- ecoMAX 613
- ecoMAX 618
- ecoMAX 622
- ecoMAX 635
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC 65kW Repair: £150-350
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC plus aquaPLUS
- ecoTEC sustain
- Home Combi Repair: £120-250
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant d.40 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