BoilerFaultCodesVaillant F.85 Fault Code
Also written as F85 — same fault.
The Vaillant F.85 fault code means: your boiler has detected that the temperature sensors are giving unusual readings, likely because they are loose or have been installed on the wrong pipes.. It appears on 27 Vaillant models in our database.
Technical description: Fault: NTCs fitted incorrectly Flow/return temperature sensors returning incorrect/implausible values. Check: Flow/return temperature sensor (correct thermal connection).
What causes the Vaillant F.85 fault?
This fault occurs when the boiler's control board detects that the temperature sensors are not sending consistent or logical readings. The most common reason is that the sensors have vibrated loose from the internal pipes or were accidentally clipped onto the wrong ones during a recent repair. Because the boiler cannot accurately track how hot the water is, it shuts down for safety.
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.85 fault code
Click a model for the full fault code list and model-specific repair notes.
- ecoFIT Pure 412 Repair: £150-250
- ecoFIT Pure 418 Repair: £120-220
- ecoFIT Pure 425 Repair: £120-220
- ecoFIT Pure 435 Repair: £90-180
- ecoFIT pure 612 Repair: £120-220
- ecoFIT pure 615 Repair: £120-220
- ecoFIT pure 618 Repair: £120-220
- ecoFIT pure 630 Repair: £120-220
- ecoFIT Pure 825 Repair: £120-220
- ecoFIT Pure 830 Repair: £120-220
- ecoFIT Pure 835 Repair: £120-220
- ecoFIT sustain 415 Repair: £90-180
- ecoFIT sustain 418 Repair: £90-180
- ecoFIT sustain 430 Repair: £90-180
- ecoFIT sustain 615 Repair: £120-220
- ecoFIT sustain 618 Repair: £120-220
- ecoFIT sustain 630 Repair: £120-220
- ecoFIT sustain 825 Repair: £90-180
- ecoFIT sustain 830 Repair: £90-180
- ecoFIT sustain 835 Repair: £90-180
- ecoTEC plus Repair: £90-180
- ecoTEC pro Repair: £100-£180
- ecoTEC sustain Repair: £90-180
- Home Combi 25 Repair: £120-220
- Home Combi 30 Repair: £120-220
- Home Combi 35 Repair: £120-220
- Home Regular 12
Source
Vaillant F.85 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 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026