BoilerFaultCodesVaillant 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 27 Vaillant models in our database.
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.
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
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like S.109: no call-out fees, no surprise bills.
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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.
- ecoFIT Pure 412
- ecoFIT Pure 418
- ecoFIT Pure 425
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830
- ecoFIT Pure 835
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoTEC plus Repair: £0-80
- ecoTEC plus 412
- ecoTEC plus 415
- ecoTEC plus 418
- ecoTEC plus 424
- ecoTEC plus 430
- ecoTEC Plus 610
- ecoTEC Plus 620 Repair: £0-80
- ecoTEC Plus 625 Repair: £0-80
- ecoTEC Plus 635 Repair: £0-80
- ecoTEC Plus 836 Repair: £0-80
- ecoTEC Plus 840 Repair: £0-80
- ecoTEC plus 940
- ecoTEC sustain Repair: £0-80
- Home Combi
- Home System
- Turbomax 242/282 1E
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.
- Vaillant ecoFIT Pure 425 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026