BoilerFaultCodesVaillant D.8 Fault Code
Also written as D8 — same fault.
The Vaillant D.8 fault code means: your boiler is receiving a signal from your thermostat or external controls asking for heat, and it is operating as it should.. It appears on 6 Vaillant models in our database.
Technical description: External controls heat demand
What causes the Vaillant D.8 fault?
This code appears when your external thermostat or smart control system sends a signal to the boiler asking it to provide heat. It is not a breakdown, but rather a normal status message confirming that the boiler has received the command and is currently working to warm your home. If your radiators are not getting hot while this is displayed, the issue likely lies with a pump or a zone valve rather than the boiler itself.
This code is most frequently seen during the autumn and winter months when heating demands are high and the thermostat is active.
DIY fix possible
Check your room thermostat or timer settings to see if you are calling for heat.Lower the temperature on your thermostat to see if the code disappears.No action is required if your heating is currently turning on and off when you expect it to.
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Gas Safe Register →Vaillant models that show the D.8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.8 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