BoilerFaultCodesVaillant F.91 Fault Code
Also written as F91 — same fault.
The Vaillant F.91 fault code means: your boiler is having trouble communicating with the internal water storage tank, which means your hot water supply may be affected or inconsistent.. It appears on 32 Vaillant models in our database.
Technical description: Sensor/actuator fault on actoSTOR module
What causes the Vaillant F.91 fault?
This fault occurs when the electrical signal from the hot water cylinder’s temperature probe is interrupted or corrupted. The most common reason is that the internal wiring of the sensor has failed or become damaged, causing a short circuit that prevents the boiler from knowing how hot your water is. In some cases, moisture ingress or loose connections at the sensor housing are the physical culprits.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Vaillant models that show the F.91 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 435
- ecoFIT pure 612
- ecoFIT pure 615
- ecoFIT pure 630
- ecoFIT Pure 825 Repair: £120-200
- ecoFIT Pure 830 Repair: £120-220
- ecoFIT Pure 835 Repair: £120-200
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 825
- ecoFIT sustain 835
- ecoTEC
- ecoTEC plus 412 Repair: £120-200
- ecoTEC plus 415 Repair: £120-200
- ecoTEC plus 418 Repair: £120-200
- ecoTEC plus 424 Repair: £120-200
- ecoTEC plus 430 Repair: £120-200
- ecoTEC Plus 610 Repair: £120-200
- ecoTEC plus 612 Repair: £120-250
- ecoTEC plus 618 Repair: £120-250
- ecoTEC Plus 620 Repair: £90-180
- ecoTEC plus 624 Repair: £120-250
- ecoTEC Plus 625 Repair: £90-180
- ecoTEC Plus 630 Repair: £90-180
- ecoTEC Plus 635 Repair: £90-180
- ecoTEC plus 637
- ecoTEC Plus 826 Repair: £150-300
- ecoTEC Plus 832 Repair: £90-180
- ecoTEC Plus 840 Repair: £90-180
- ecoTEC Plus 938 Repair: £150-300
- ecoTEC Pro / ecoTEC Plus
Source
Vaillant F.91 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 835 manual (PDF)
- Vaillant ecoFIT pure 630 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026