S.33 High

Vaillant 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.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

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.

Typical repair cost £120-280 (including labour)

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.33 fault code

Click a model for the full fault code list and model-specific repair notes.

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026