D.039 Low

Vaillant D.039 Fault Code

Also written as D039 — same fault.

The Vaillant D.039 fault code means: your boiler is struggling to read the temperature from your solar water heating system, meaning it cannot accurately tell if your stored solar water is hot enough to use.. It appears on 7 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: Solar inlet temp. actual value
Typical repair cost £120-200 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Vaillant models that show the D.039 fault code

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

Source

Vaillant D.039 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