S.17 Medium

Vaillant S.17 Fault Code

Also written as S17 — same fault.

The Vaillant S.17 fault code means: your boiler has detected that the water is not flowing correctly through the heating system, likely because a valve is closed or there is a blockage.. It appears on 40 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: Water pump return water

What causes the Vaillant S.17 fault?

This fault occurs when the boiler's sensors detect that water is returning to the unit too quickly without losing heat, which usually means the water isn't circulating through your radiators properly. The most common reason is that the valves on your radiators are closed or there is a physical blockage in the pipework preventing the flow. It can also be triggered if the boiler's internal pump is struggling to push water around the system.

This code often appears at the end of autumn when heating systems are switched back on after being idle for several months, which can cause internal parts or valves to seize up.

Typical repair cost £100-£350 (including labour)

DIY fix possible

Check that at least two or three radiator valves (TRVs) are fully open to allow water flow.Ensure the external heating bypass valve, if visible, is not completely shut.Gently bleed your radiators to ensure there is no trapped air causing a blockage.

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Vaillant models that show the S.17 fault code

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

Source

Vaillant S.17 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