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 9 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.
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 ecoTEC Plus 610 manual (PDF)
- Vaillant ecoTEC Plus 938 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026