D.33 Medium

Vaillant D.33 Fault Code

Also written as D33 — same fault.

The Vaillant D.33 fault code means: your boiler is struggling to match the correct fan speed needed to safely clear exhaust gases, which may lead to the system pausing for safety.. It appears on 28 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Fan speed target value

What causes the Vaillant D.33 fault?

This fault occurs when the boiler's control board sends a signal to the fan but does not receive the correct speed reading in return. The most common reason is that the fan motor is failing or the electrical connections between the fan and the main circuit board have become loose or damaged. Without the fan spinning at exactly the right speed, the boiler cannot safely manage the airflow required for combustion.

Typical repair cost £180-450 (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 7 July 2026