BoilerFaultCodesVaillant d.08 Fault Code
Also written as D08 — same fault.
The Vaillant d.08 fault code means: this code is simply an information display showing the current signal from your wall thermostat, letting you know if your heating is correctly receiving a request to turn on or off.. It appears on 22 Vaillant models in our database.
Technical description: Status of the 230V thermostat: 0=Room thermostat open (no heat requirement), 1=Room thermostat closed (heat requirement)
DIY fix possible
Check your wall thermostat to see if it is set to a higher temperature than the current room temperatureEnsure your programmer or digital timer is set to the 'On' position for heatingIf the boiler is not firing when expected, change the batteries in your wireless thermostat if applicable
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the d.08 fault code
Click a model for the full fault code list and model-specific repair notes.
- aquaPLUS
- ecoFIT pure Repair: £0-120
- ecoFIT Pure 412
- ecoFIT Pure 418 Repair: £0-120
- ecoFIT Pure 425 Repair: £0-120
- ecoFIT Pure 435 Repair: £0-120
- ecoFIT Pure 835 Repair: £0-120
- ecoTEC plus
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 65
- ecoTEC Plus 832
- ecoTEC Plus 836
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus Aquaplus
- ecoTEC plus System
- ecoTEC sustain
- Home System Repair: £0-120
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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026