BoilerFaultCodesVaillant d.08 Fault Code
Also written as D08 — same fault.
The Vaillant d.08 fault code means: your boiler is receiving a standard signal from your thermostat or timer to turn on the heating or hot water, and it is simply confirming that it has understood and is starting the process.. It appears on 18 Vaillant models in our database.
Technical description: External controls heat demand
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the d.08 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 418 Repair: £0-120
- ecoFIT Pure 425 Repair: £0-120
- ecoFIT Pure 435 Repair: £0-120
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835 Repair: £0-120
- ecoTEC plus aquaPLUS
- ecoTEC sustain
- Home Combi
- Home System Repair: £0-120
- 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.08 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 ecoTEC plus aquaPLUS manual (PDF)
- Vaillant ecoTEC sustain manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026