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 39 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
Find a Gas Safe engineer
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A cover plan would handle faults like S.98: no call-out fees, no surprise bills.
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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.
- Atmomax Repair: £90-180
- ecoFIT Pure 412 Repair: £100-200
- ecoFIT Pure 418 Repair: £120-200
- ecoFIT Pure 425 Repair: £120-200
- ecoFIT Pure 435
- ecoFIT Pure 825
- ecoFIT Pure 830 Repair: £100-200
- ecoFIT Pure 835
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoTEC
- ecoTEC 65kW
- ecoTEC exclusive Repair: £100-220
- ecoTEC exclusive 627
- ecoTEC plus 412 Repair: £100-250
- ecoTEC plus 415 Repair: £100-250
- ecoTEC plus 418 Repair: £100-250
- ecoTEC plus 424 Repair: £100-250
- ecoTEC plus 430 Repair: £100-250
- ecoTEC Plus 610 Repair: £100-250
- ecoTEC Plus 615 Repair: £90-180
- ecoTEC Plus 620 Repair: £90-180
- ecoTEC Plus 625 Repair: £90-180
- ecoTEC Plus 630 Repair: £90-180
- ecoTEC Plus 635 Repair: £90-180
- ecoTEC plus 637
- ecoTEC Plus 826
- ecoTEC Plus 832 Repair: £90-180
- ecoTEC Plus 840 Repair: £90-180
- ecoTEC Plus 938 Repair: £90-180
- ecoTEC Pro Repair: £0-120
- ecoTEC Pro / ecoTEC Plus
- ecoTEC Pro 24 Repair: £0-120
- ecoTEC Pro 28 Repair: £0-120
- ecoTEC Pro 30 Repair: £80-150
- ecoTEC sustain Repair: £90-180
- Home Combi
- Home System Repair: £90-180
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 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026