BoilerFaultCodesVaillant d.28 Fault Code
Also written as D28 — same fault.
The Vaillant d.28 fault code means: your boiler is struggling to communicate with an external accessory, such as an additional pump or timer, which might be preventing your heating or hot water from working correctly.. It appears on 16 Vaillant models in our database.
Technical description: Function of relay 2 (multi-functional module)
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.28 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £100-250
- ecoFIT Pure 418 Repair: £80-160
- ecoFIT Pure 425 Repair: £90-180
- ecoFIT Pure 435 Repair: £90-180
- ecoFIT Pure 825 Repair: £120-250
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £90-180
- ecoTEC 65kW Repair: £150-300
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC sustain
- Home Combi Repair: £120-250
- Home System Repair: £80-120
Source
Vaillant d.28 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