How much does it cost to fix Vaillant error code S.3?

Typical repair cost for the Vaillant S.3 fault is £0-120 including labour.

S.3 Low

Vaillant S.3 Fault Code

Q: What does the Vaillant S.3 fault code mean?

Your boiler is currently trying to start the ignition process to provide heating, which is a normal part of the startup sequence and not necessarily a fault unless it remains stuck on this screen.

Also written as S3 — same fault.

The Vaillant S.3 fault code means: your boiler is currently trying to start the ignition process to provide heating, which is a normal part of the startup sequence and not necessarily a fault unless it remains stuck on this screen.. It appears on 26 Vaillant models in our database.

Technical description: Ignition sequence (heating operation)

What causes the Vaillant S.3 fault?

This status occurs when the boiler begins the physical process of lighting the burner to heat your radiators. It is a natural part of the start-up cycle where the system checks components and prepares the spark to ignite the gas. If the boiler stays on this code without moving forward, it usually means the system is struggling to successfully establish or detect a flame.

Typical repair cost £0-120 (including labour)

DIY fix possible

Check if the display changes to a different code after a few secondsMonitor for any lockout codes if the boiler fails to fireEnsure the gas supply is turned on at the meterPress the reset button if the boiler becomes stuck

Find a Gas Safe engineer

Search the official register for a qualified engineer in your area.

Gas Safe Register →

Vaillant models that show the S.3 fault code

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

Source

Vaillant S.3 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