d.39 Low

Vaillant d.39 Fault Code

Also written as D39 — same fault.

The Vaillant d.39 fault code means: your boiler is receiving an unusual signal from the sensor that monitors the temperature of the optional solar water heating system connected to your home.. It appears on 11 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
Typical repair cost £120-250 (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.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 31 August 2026