S.6 High

Vaillant S.6 Fault Code

Also written as S6 — same fault.

The Vaillant S.6 fault code means: your boiler is trapped in a cooling-down cycle where the fan keeps running longer than necessary, preventing it from starting a new heating cycle.. It appears on 26 Vaillant models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: Heating system fan over run

What causes the Vaillant S.6 fault?

This fault occurs when the boiler's control board fails to receive a signal that the fan has slowed down or stopped after a heating cycle, causing it to stay in a permanent cooling mode. The most common reason is a malfunctioning relay on the main circuit board or a faulty fan that is sending incorrect speed data back to the system. While the boiler is stuck in this 'over run' phase, it will refuse to ignite for central heating or hot water because it believes it hasn't finished cooling down.

Typical repair cost £150-450 (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 S.6 fault code

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

Source

Vaillant S.6 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 27 April 2026