S.26 Medium

Vaillant S.26 Fault Code

Also written as S26 — same fault.

The Vaillant S.26 fault code means: your boiler is staying in a cooling-down mode because the internal fan is running for longer than it should after heating your water.. It appears on 38 Vaillant models in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Hot–water fan over run

What causes the Vaillant S.26 fault?

This issue occurs when the boiler's internal fan fails to stop spinning after the heating cycle has finished. It is usually caused by a faulty control board sending power to the fan when it shouldn't, or the fan itself failing to report that it has slowed down correctly. Because the boiler is waiting for the 'cool down' phase to end, it will stay in this standby mode and won't start the next time you need heat.

Typical repair cost £150-350 (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.26 fault code

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

Source

Vaillant S.26 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