BoilerFaultCodesVaillant D.000 Fault Code
Also written as D000 — same fault.
The Vaillant D.000 fault code means: your boiler is currently set to run at a lower power level than its maximum capacity to save energy or match your home's heat demand.. It appears on 12 Vaillant models in our database.
Technical description: Partial heat load (Diagnostics menu)
What causes the Vaillant D.000 fault?
This status appears because the boiler’s maximum output has been manually limited during installation or a service to better suit the size of your property. It is not a mechanical failure, but rather a configuration setting that ensures the unit doesn't use more gas than necessary to heat your radiators. It prevents the system from 'short cycling,' which occurs when a boiler is too powerful for the home it is heating.
DIY fix possible
Press the 'Menu' button on the control panelNavigate to the diagnostics settings (D.00) using the +/- buttonsReview if the output kW matches your home's requirements, but usually, this setting should be left to a professional during installation
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Vaillant models that show the D.000 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.000 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