D.51 Medium

Vaillant D.51 Fault Code

Also written as D51 — same fault.

The Vaillant D.51 fault code means: your boiler is signalling a setup issue related to the fan's speed range, meaning it is not operating at its intended maximum performance level.. It appears on 25 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: Offset for maximum speed

What causes the Vaillant D.51 fault?

This fault occurs when the boiler's control board detects that the internal fan is unable to reach its specified maximum speed setting. It is usually caused by a communication error between the main circuit board and the fan, or an incorrect configuration value stored in the boiler's memory. This means the appliance cannot safely ramp up to its full heating power, though it may still operate at a lower capacity.

Typical repair cost £90-180 (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.51 fault code

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

Source

Vaillant D.51 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