D.73 Low

Vaillant D.73 Fault Code

Also written as D73 — same fault.

The Vaillant D.73 fault code means: your boiler is notifying you that it has adjusted the temperature settings for its pre-heat function to ensure hot water is ready when needed.. It appears on 13 Vaillant models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Off set for warm start target value

What causes the Vaillant D.73 fault?

This occurs when the boiler’s internal software automatically shifts the target temperature for the 'warm start' feature to ensure the heat exchanger stays ready for use. It is usually triggered by the control system detecting that the internal water temperature has dropped below a specific threshold required for instant delivery. This is a self-correcting logic adjustment rather than a mechanical failure.

This notification is more frequent during colder months when lower ambient temperatures cause the water inside the boiler to cool down more rapidly between uses.

Typical repair cost £0-120 (including labour)

DIY fix possible

No action is typically required as this is often an advisory status code rather than a hard fault.If hot water temperatures feel inconsistent, check your boiler's hot water temperature dial/setting.Perform a simple reset by pressing the 'flame' or 'reset' button to see if the status clears.

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Vaillant models that show the D.73 fault code

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

Source

Vaillant D.73 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