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 temperature sensor in your hot water cylinder, which means it may not be able to heat your taps or shower correctly.. It appears on 11 Vaillant models in our database.
Technical description: Short circuit: Cylinder temperature sensor
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 612 Repair: £120-200
- ecoFIT pure 615 Repair: £120-200
- ecoFIT pure 618 Repair: £120-200
- ecoFIT pure 630 Repair: £120-200
- ecoFIT Pure 825 Repair: £120-200
- ecoFIT Pure 830 Repair: £120-220
- ecoFIT Pure 835 Repair: £120-200
- ecoFIT sustain 615 Repair: £120-200
- ecoFIT sustain 618 Repair: £120-200
- ecoFIT sustain 630 Repair: £120-200
- ecoTEC plus Repair: £100-300
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 630 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