BoilerFaultCodesVaillant S.33 Fault Code
Also written as S33 — same fault.
The Vaillant S.33 fault code means: your boiler is struggling to start because it cannot confirm that the fan is pushing enough air through the flue to safely clear fumes.. It appears on 23 Vaillant models in our database.
Technical description: Air pressure switch (in fan outlet) tube and air/flue gas system. Check hoses are not obstructed and correctly fitted and not damaged. Check air pressure switch or replace. Check flue restrictor corresponds to flue length.
What causes the Vaillant S.33 fault?
This fault occurs when the boiler's safety sensors cannot detect sufficient airflow to safely exhaust burnt gases to the outside. The most common reasons are a failing fan that isn't spinning fast enough, a blockage in the flue pipe, or thin rubber tubes inside the unit becoming perished or blocked with moisture.
This fault is more common during periods of high winds, as strong gusts can create back-pressure in the flue that prevents the air pressure switch from closing correctly.
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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A cover plan would handle faults like S.33: no call-out fees, no surprise bills.
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Vaillant models that show the S.33 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 435 Repair: £150-300
- ecoFIT Pure 825 Repair: £150-300
- ecoFIT Pure 830 Repair: £150-300
- ecoFIT Pure 835 Repair: £150-300
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoTEC exclusive 627
- ecoTEC Plus 620 Repair: £150-300
- ecoTEC Plus 625 Repair: £150-300
- ecoTEC Plus 635 Repair: £150-300
- ecoTEC Plus 840 Repair: £150-300
- ecoTEC plus aquaPLUS Repair: £120-350
- Home Combi
- Home System Repair: £120-350
- Thermocompact Repair: £120-280
- Turbomax 824/828 Repair: £150-350
- Turbomax Plus Repair: £120-300
- Turbomax Plus 824/828E Repair: £150-350
- Turbomax Plus 828 Repair: £100-280
- Turbomax Plus 828 E Repair: £150-350
- Turbomax Plus VUW 242E Repair: £150-350
- Turbomax Plus VUW 242EH Repair: £150-350
Source
Vaillant S.33 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 ecoFIT sustain 630 manual (PDF)
- Vaillant ecoTEC exclusive 627 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026