S.25 Low

Vaillant S.25 Fault Code

Also written as S25 — same fault.

The Vaillant S.25 fault code means: your boiler is busy prepping for a heating cycle by checking its internal sensors and clearing the flue, but if it stays stuck on this display, it may be failing to light.. 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 water pump running

What causes the Vaillant S.25 fault?

This code appears when the boiler begins its startup sequence, spinning up the fan and water pump to ensure the system is safe to ignite. If the boiler becomes stuck on this screen, it usually means the internal controls are waiting for a signal to proceed that isn't arriving. In many cases, this is caused by a minor electrical communication issue or a component failing to reach the necessary speed to trigger the next step.

Typical repair cost £0-120 (including labour)

DIY fix possible

Wait 5-10 minutes to see if the boiler progresses to a fire-up sequenceEnsure your thermostat is calling for heat (turned up)Reset the boiler using the button with the flame/cross icon

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Vaillant models that show the S.25 fault code

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

Source

Vaillant S.25 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