BoilerFaultCodesVaillant D.34 Fault Code
Also written as D34 — same fault.
The Vaillant D.34 fault code means: your boiler is struggling to accurately measure how fast its internal fan is spinning, which is preventing the system from heating correctly.. It appears on 37 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 get a consistent reading of how fast the fan is spinning. The most common reason is a communication failure between the electronic heart of the boiler and the fan motor, often due to loose wiring or a failing motor.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
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Vaillant models that show the D.34 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £200-£450
- ecoFIT Pure 418
- ecoFIT Pure 425 Repair: £180-450
- ecoFIT Pure 435
- ecoFIT Pure 825 Repair: £180-350
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoMAX
- ecoMAX 613
- ecoMAX 618
- ecoMAX 622
- ecoMAX 635
- ecoMAX 824E
- ecoMAX 828E
- Ecomax Condensing 600/800 series
- ecoMAX System 612
- ecoTEC Repair: £250-480
- ecoTEC 65kW Repair: £180-350
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 840
- ecoTEC plus aquaPLUS Repair: £0-120
- ecoTEC Pro / ecoTEC Plus Repair: £150-300
- ecoTEC sustain
- Home Combi Repair: £250-450
- Home System Repair: £0-120
- Thermocompact Repair: £180-350
- Turbomax 824/828 Repair: £150-300
- Turbomax Plus Repair: £180-350
- Turbomax Plus 824/828E Repair: £180-350
- Turbomax Plus 828 Repair: £150-300
- Turbomax Plus 828 E Repair: £180-350
- Turbomax Plus VUW 242E Repair: £150-350
- Turbomax Plus VUW 242EH Repair: £150-350
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.
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
- Vaillant ecoMAX System 612 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026