BoilerFaultCodesVaillant D.34 Fault Code
Also written as D34 — same fault.
The Vaillant D.34 fault code means: your boiler is having trouble controlling the speed of its internal fan, which is preventing it from starting up safely.. It appears on 28 Vaillant models in our database.
Technical description: Fan speed actual value
What causes the Vaillant D.34 fault?
This fault occurs when the boiler's control board cannot accurately track or regulate how fast the fan is spinning. The most common reason is a mechanical failure within the fan motor or a communication break in the wiring, which prevents the boiler from safely clearing waste gases. Because the fan is a critical safety component, the system will prevent the boiler from firing up if the speed is inconsistent.
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.34 fault code
Click a model for the full fault code list and model-specific repair notes.
- aquaPLUS Repair: £0-120
- Atmomax Repair: £200-£450
- Combi Compact 242EB Repair: £250-450
- Combi Compact 282EB Repair: £180-£350
- ecoFIT pure
- ecoFIT Pure 412 Repair: £200-350
- ecoFIT Pure 418
- ecoFIT Pure 425 Repair: £180-450
- ecoFIT Pure 435
- ecoFIT Pure 835
- Ecomax Condensing 600/800 series
- ecoTEC Repair: £250-480
- 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 Regular Repair: £250-450
- ecoTEC plus System
- ecoTEC sustain
- Home System Repair: £0-120
Source
Vaillant D.34 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