F.33 High

Vaillant F.33 Fault Code

Also written as F33 — same fault.

The Vaillant F.33 fault code means: your boiler has stopped working because it cannot confirm that harmful exhaust fumes are being safely cleared away through the flue pipe.. It appears on 11 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: (837/637 only) | Main heat exchanger anti freeze protection mode due to no switching signal from the air pressure switch | Check air pressure switch, fan, fan supply or flue system

What causes the Vaillant F.33 fault?

This fault occurs when the boiler's internal safety sensor doesn't detect the necessary airflow required to safely expel exhaust gases. The most common reasons are a failing fan that cannot spin fast enough, a blocked or damaged flue pipe, or a faulty sensor that is incorrectly reporting a lack of pressure.

This fault is more frequent during periods of high winds or extreme cold, as wind gusts can disrupt flue pressure and ice buildup can physically block the air intake.

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 F.33 fault code

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

Source

Vaillant F.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 7 July 2026