BoilerFaultCodesVaillant S.34 Fault Code
Also written as S34 — same fault.
The Vaillant S.34 fault code means: your boiler has detected very cold temperatures and is running its internal pump to prevent the water inside from freezing.. It appears on 46 Vaillant models in our database.
Technical description: Frostprotectionmodeactive
What causes the Vaillant S.34 fault?
This fault occurs when the boiler’s internal sensors detect that the water temperature has dropped to a level where there is a genuine risk of the heat exchanger freezing. It typically happens because the boiler is located in a very cold area, such as a garage or outbuilding, or because a faulty sensor is sending an incorrect freezing signal to the control board. When this happens, the boiler pauses operation as a safety measure to prevent the internal pipework from cracking.
This fault is almost exclusively seen during the winter months when external temperatures drop near or below freezing.
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 S.34 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £0-120
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- 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
- ecoTEC 65kW
- ecoTEC exclusive
- ecoTEC Plus 610
- ecoTEC Plus 615
- ecoTEC Plus 620
- ecoTEC Plus 625
- ecoTEC Plus 630
- ecoTEC Plus 635
- ecoTEC Plus 826
- ecoTEC Plus 840
- ecoTEC Plus 938
- ecoTEC plus aquaPLUS
- ecoTEC Pro
- ecoTEC Pro / ecoTEC Plus Repair: £90-180
- ecoTEC Pro 24
- ecoTEC Pro 28
- ecoTEC Pro 30
- ecoTEC sustain
- Home Combi
- Home System
- Thermocompact
- Turbomax 824/828
- Turbomax Plus
- Turbomax Plus 824/828E
- Turbomax Plus 828 Repair: £0-60
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant S.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 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoMAX manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026