d.09 Low

Vaillant d.09 Fault Code

Also written as D09 — same fault.

The Vaillant d.09 fault code means: your boiler is simply displaying the target temperature being requested by your external room thermostat or smart control system; this is a status message rather than a breakdown.. It appears on 9 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: Heating flow set target temperature that is set on the eBUS room thermostat

DIY fix possible

Check your room thermostat to see if it is calling for heatAdjust your thermostat settings or schedule if the temperature is higher or lower than expectedNo action is required if your heating is performing normally as this is a diagnostic status

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Vaillant models that show the d.09 fault code

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

Source

Vaillant d.09 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