BoilerFaultCodesVaillant D.3 Fault Code
Also written as D3 — same fault.
The Vaillant D.3 fault code means: your boiler is currently showing you the live temperature of the hot water it is producing, which is a normal status message rather than an error.. It appears on 5 Vaillant models in our database.
Technical description: Hot water flow temperature reading
What causes the Vaillant D.3 fault?
This is a status code rather than a mechanical failure, occurring when the boiler sensor detects a demand for hot water. It simply means the internal computer is measuring and displaying the temperature of the water currently being heated for your taps or shower. You will see this code every time you turn on a hot tap as part of the boiler's standard operating sequence.
DIY fix possible
No action is required as this is a status code, not a fault.If the boiler is stuck on this code and not providing heat, try resetting the boiler.Ensure your hot water taps are fully closed.
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Gas Safe Register →Vaillant models that show the D.3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.3 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