BoilerFaultCodesVaillant D.28 Fault Code
Also written as D28 — same fault.
The Vaillant D.28 fault code means: your boiler has detected a communication problem or a wiring fault with an external control accessory, such as an additional pump or zone valve.. It appears on 25 Vaillant models in our database.
Technical description: Switching accessory relay 2
What causes the Vaillant D.28 fault?
This fault occurs when the boiler's main control board loses communication with a specific internal relay that manages external components like your central heating pump. It is usually caused by a loose connection in the wiring harness or a component on the circuit board that has failed over time. In some cases, an electrical surge or moisture ingress may have damaged the sensitive electronic links between these parts.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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.28 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £100-250
- Combi Compact 242EB Repair: £100-280
- Combi Compact 282EB Repair: £120-280
- ecoFIT pure Repair: £100-280
- ecoFIT Pure 412 Repair: £120-250
- ecoFIT Pure 418 Repair: £80-160
- ecoFIT Pure 425 Repair: £90-180
- ecoFIT Pure 435 Repair: £90-180
- ecoFIT Pure 835 Repair: £90-180
- ecoTEC Repair: £120-250
- 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 Regular Repair: £100-250
- ecoTEC plus System
- ecoTEC sustain
- 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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026