BoilerFaultCodesVaillant D.4 Fault Code
Also written as D4 — same fault.
The Vaillant D.4 fault code means: your boiler is currently showing the live temperature reading of the sensor responsible for keeping your hot water pre-heated.. It appears on 4 Vaillant models in our database.
Technical description: Current temperature for warm stat sensor
What causes the Vaillant D.4 fault?
This isn't actually a fault, but a live data reading showing the temperature of the internal sensor that manages your pre-heated hot water. It appears when the boiler is actively monitoring the heat of the water held inside the heat exchanger to ensure you get hot water quickly when you turn on a tap. If you see this, it simply means the 'Warmstart' function is active and the system is checking the current heat level.
DIY fix possible
No action required; this is an information code rather than a fault.Wait for the pre-heat cycle to finish and the display to return to normal.If you wish to stop this occurring, find the 'Comfort' mode setting and turn it off.
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Vaillant models that show the D.4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vaillant D.4 fault code information on this page is sourced and cross-checked against the official Vaillant service / installation manuals for each affected model.
- Vaillant ecoTEC manual (PDF)
- Vaillant ecoTEC 65kW manual (PDF)
- Vaillant ecoTEC Pro / ecoTEC Plus manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026