BoilerFaultCodesVaillant D.9 Fault Code
Also written as D9 — same fault.
The Vaillant D.9 fault code means: your boiler is receiving a specific instruction from an external controller or thermostat telling it exactly what temperature to heat the water to.. It appears on 5 Vaillant models in our database.
Technical description: Flow target temperature from external analogue regulator terminal
What causes the Vaillant D.9 fault?
This is not actually a fault, but an information message triggered when an external smart thermostat or plug-in controller takes over the boiler's temperature settings. It occurs when your external controls send a specific voltage signal to the boiler, telling it to override the manual dials and heat to a precise target. If your heating isn't working while this is displayed, the issue usually lies with a setting on your room thermostat or a communication error between the two devices.
DIY fix possible
Check if your external room thermostat or smart home hub is set to a very low temperatureEnsure your external controls have power and are connected to your Wi-Fi if applicableIncrease the temperature setting on your external thermostat to see if the boiler fires up
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Gas Safe Register →Vaillant models that show the D.9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.9 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