S.109 Low

Vaillant S.109 Fault Code

Also written as S109 — same fault.

The Vaillant S.109 fault code means: your boiler has temporarily stopped working because it has entered a standby mode, usually while it waits for a signal from your thermostat or because it needs to take a short break before restarting.. It appears on 11 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: Product'sstandbymodeactivated

What causes the Vaillant S.109 fault?

This code appears when the boiler reaches its target temperature or doesn't receive a call for heat from your home controls, causing it to pause operations for several minutes to prevent constant on-off cycling. It is often triggered by an external thermostat that has satisfied the room temperature or has lost its wireless connection to the boiler. This 'anti-cycling' state is a protective feature designed to save energy and reduce wear on internal components.

This status is more frequently observed in the spring and autumn when the weather is milder, as rooms reach their target temperature quickly and the thermostat cycles the boiler off more often.

Typical repair cost £0-80 (including labour)

DIY fix possible

Check that your room thermostat is turned up and calling for heatEnsure your programmer or heating app is set to an 'On' periodWait up to 15 minutes to see if the boiler restarts automatically

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Vaillant models that show the S.109 fault code

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

Source

Vaillant S.109 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 31 August 2026