BoilerFaultCodesVaillant S.96 Fault Code
Also written as S96 — same fault.
The Vaillant S.96 fault code means: your boiler is performing a self-diagnostic check on its internal temperature sensors and has temporarily paused heating until it confirms everything is working correctly.. It appears on 15 Vaillant models in our database.
Technical description: Return temperature sensor self-test: Return sensor test running, heating demands are blocked.
What causes the Vaillant S.96 fault?
This occurs when the boiler's control system notices an unusual discrepancy between the temperature readings of the flow and return pipes. To ensure the heat exchanger doesn't overheat, the boiler pauses operation to run a quick test and verify that the internal sensors are reporting accurate data. It is often triggered by air bubbles trapped in the system or a pump that is beginning to struggle with water circulation.
This can be more frequent in autumn when the heating is turned on for the first time after summer, as air often settles in the pipework during the warmer months.
DIY fix possible
Wait up to 15 minutes for the automated test to completeSwitch the boiler off and back on at the fused spur switch to trigger a restartEnsure all radiator valves are open to allow for proper water circulation
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Vaillant models that show the S.96 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £100-200
- ecoFIT Pure 418
- ecoFIT Pure 425 Repair: £100-200
- ecoFIT Pure 435 Repair: £120-250
- ecoFIT Pure 825
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835
- ecoTEC exclusive Repair: £0-120
- ecoTEC plus
- ecoTEC plus VU GB 656/4-5 A
- ecoTEC pro Repair: £0-120
- ecoTEC sustain Repair: £0-180
- Home Combi 25 Repair: £100-200
- Home Combi 30 Repair: £100-200
- Home Combi 35 Repair: £100-200
Source
Vaillant S.96 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