BoilerFaultCodesVaillant S.03 Fault Code
Also written as S03 — same fault.
The Vaillant S.03 fault code means: your boiler is currently trying to light the flame to start your heating, which is a normal part of its startup sequence.. It appears on 19 Vaillant models in our database.
Technical description: Heating mode: Ignition. The ignition for heating mode is activated.
What causes the Vaillant S.03 fault?
This code appears when the boiler is actively trying to light the internal flame to provide heat for your radiators. It is a normal part of the startup process where the boiler sends a spark to the burner to get the system running. If this code stays on the screen for a long time without the heating starting, it may indicate a struggle to establish a stable flame.
This status is most frequently seen during the autumn and winter months when the heating system is first being called upon to run after being idle over summer.
DIY fix possible
Check that your gas supply is turned on at the meterEnsure your thermostat is calling for heatWait a few minutes to see if the status changes to a normal running code
Find a Gas Safe engineer
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Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like S.03: no call-out fees, no surprise bills.
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Vaillant models that show the S.03 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
- ecoTEC pro
- ecoTEC sustain
- Home Combi 25
- Home Combi 30
- Home Combi 35
- Turbomax 824/828
- Turbomax Plus Repair: £120-250
- Turbomax Plus 824/828E
- Turbomax Plus 828
- Turbomax Plus 828 E
- Turbomax Plus VUW 242E
- Turbomax Plus VUW 242EH
Source
Vaillant S.03 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 435 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026