BoilerFaultCodesVaillant D.40 Fault Code
Also written as D40 — same fault.
The Vaillant D.40 fault code means: your boiler is displaying the temperature it is currently aiming for rather than a fault, which is a normal status message and not a breakdown.. It appears on 37 Vaillant models in our database.
Technical description: Flow temperature target value
What causes the Vaillant D.40 fault?
This code appears when the boiler's internal computer calculates the specific temperature it needs to send to your radiators to meet your heating demands. It is physically triggered by the system's sensors and controls communicating the required heat level based on your thermostat settings. Because this is a standard status update rather than a failure, your boiler will continue to operate as normal.
DIY fix possible
Check if your thermostat or external controls are set higher than the current room temperatureWait for the boiler to complete its current heating cyclePress the 'i' button to exit the diagnostic information menu if you entered it accidentally
Find a Gas Safe engineer
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A cover plan would handle faults like D.40: no call-out fees, no surprise bills.
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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 824E
- ecoMAX 828E
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC
- ecoTEC 65kW Repair: £150-350
- 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 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 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026