BoilerFaultCodesVaillant d.27 Fault Code
Also written as D27 — same fault.
The Vaillant d.27 fault code means: your boiler is having trouble communicating with an add-on device, such as an external pump or a motorized valve, which may prevent your heating or hot water from turning on correctly.. It appears on 16 Vaillant models in our database.
Technical description: Functionofrelay1 (multi-functionalmodule)
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.27 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £250-450
- ecoFIT Pure 418 Repair: £180-450
- ecoFIT Pure 425 Repair: £120-350
- ecoFIT Pure 435 Repair: £90-220
- ecoFIT Pure 825 Repair: £120-250
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £120-£280
- ecoTEC 65kW Repair: £120-250
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC sustain
- Home Combi Repair: £150-250
- Home System Repair: £80-160
Source
Vaillant d.27 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 ecoTEC 65kW manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026