d.79 Low

Vaillant d.79 Fault Code

Also written as D79 — same fault.

The Vaillant d.79 fault code means: your boiler is currently performing a routine high-temperature cycle to ensure your hot water system is hygienically clean and safe from bacteria.. It appears on 1 Vaillant model 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: Anti- legionella protection mode activated 1 = no, 0 = yes

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.79 fault code

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

Source

Vaillant d.79 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