BoilerFaultCodesVaillant S.98 Fault Code
Also written as S98 — same fault.
The Vaillant S.98 fault code means: your boiler is temporarily paused because it has detected a potential issue with its internal sensors while performing a self-test.. It appears on 15 Vaillant models in our database.
Technical description: Flow-/Return-sensor check is running, demand (DHW or heating) is blocked
What causes the Vaillant S.98 fault?
This fault occurs when the boiler performs a safety check and notices that the temperature sensors for the water going out and coming back are giving conflicting or illogical readings. The most common reason is that one of the sensors has developed a fault or is starting to wear out, preventing the boiler from accurately monitoring its own performance. It pauses the heating process to ensure it doesn't overheat or run inefficiently based on incorrect data.
DIY fix possible
Wait for 15-30 minutes to see if the status clears automatically after the self-test finishesSwitch the boiler off at the fused spur switch, wait 30 seconds, and switch it back onPress the 'Reset' button (indicated by a flame with a line through it) for five seconds
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Vaillant models that show the S.98 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 Repair: £120-200
- ecoFIT Pure 425 Repair: £120-200
- ecoFIT Pure 435 Repair: £100-200
- ecoFIT Pure 825
- ecoFIT Pure 830 Repair: £100-200
- ecoFIT Pure 835
- ecoTEC exclusive Repair: £100-220
- ecoTEC plus
- ecoTEC plus VU GB 656/4-5 A
- ecoTEC pro Repair: £0-120
- ecoTEC sustain Repair: £90-180
- Home Combi 25
- Home Combi 30
- Home Combi 35
Source
Vaillant S.98 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