S.02 Low

Vaillant S.02 Fault Code

Also written as S02 — same fault.

The Vaillant S.02 fault code means: your boiler is currently preparing for heating by circulating water, but it has not yet fired the burner to create heat.. It appears on 31 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: Heating mode error – water pump running

What causes the Vaillant S.02 fault?

Strictly speaking, this is a status message rather than a mechanical failure, indicating that the boiler is in a pre-heating phase. It occurs when the control system has told the pump to move water throughout the radiators, but the burner hasn't actually switched on yet. This often happens if an external thermostat is calling for heat or if the system is simply performing a routine check before ignition.

This code is most frequently seen in the autumn when homeowners first turn their heating back on after the summer and the system takes a moment to prime itself.

Typical repair cost £0-80 (including labour)

DIY fix possible

Check that your room thermostat or programmer is set to a high enough temperature to trigger heatEnsure all radiator valves are openWait up to 10 minutes to see if the code changes to a 'running' status (S.04)

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Vaillant models that show the S.02 fault code

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

Source

Vaillant S.02 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