S.05 Low

Vaillant S.05 Fault Code

Also written as S05 — same fault.

The Vaillant S.05 fault code means: your boiler is currently cooling itself down after a period of operation, which is a normal process and not actually a fault code.. It appears on 27 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: Fan and pump over run (837/637 only)

What causes the Vaillant S.05 fault?

This status occurs because your boiler needs to move heat away from its internal components after it has finished heating your home or hot water. The fan and pump continue to run for a short time to safely vent residual heat and prevent the unit from overheating. Once the internal temperature has dropped to a safe level, the boiler will return to its normal standby mode.

DIY fix possible

Wait for approximately 5 to 10 minutes for the cycle to complete.Ensure your room thermostat or programmer is calling for heat if the boiler does not restart.Check that your radiator valves are open to allow heat to dissipate.

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Vaillant models that show the S.05 fault code

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

Source

Vaillant S.05 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