d.04 Low

Vaillant d.04 Fault Code

Also written as D04 — same fault.

The Vaillant d.04 fault code means: your boiler is displaying the current temperature of the water inside your hot water storage cylinder; this is a status information code rather than a system failure.. It appears on 10 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: Water temperature in the cylinder

DIY fix possible

This fault may be fixable without an engineer. Check your manual first.

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Vaillant models that show the d.04 fault code

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

Source

Vaillant d.04 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