D.33 High

Vaillant D.33 Fault Code

Also written as D33 — same fault.

The Vaillant D.33 fault code means: your boiler's fan is not reaching the correct speed required to safely clear exhaust gases, preventing the heater from starting up.. It appears on 27 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: Fan speed target value

What causes the Vaillant D.33 fault?

This fault occurs when the boiler's control system attempts to start the fan, but the fan fails to spin at the required speed to safely clear waste gases. The most common reasons include a mechanical failure of the fan motor, a buildup of debris blocking the rotation, or a communication error from the main circuit board. Because the boiler cannot guarantee a safe flow of air, it will refuse to ignite as a safety precaution.

Typical repair cost £250-450 (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 D.33 fault code

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

Source

Vaillant D.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