d.39 Low

Vaillant d.39 Fault Code

Also written as D39 — same fault.

The Vaillant d.39 fault code means: your boiler is reporting a signal from the solar thermal panel sensor, which is informing the system about the current temperature of the water heated by sunlight.. It appears on 9 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 solar circuit

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.39 fault code

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

Source

Vaillant d.39 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