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 19 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.
Find a Gas Safe engineer
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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.
- Atmomax Repair: £90-180
- ecoFIT Pure 412 Repair: £150-250
- ecoFIT Pure 418 Repair: £120-220
- ecoFIT Pure 425 Repair: £120-220
- ecoFIT Pure 435 Repair: £120-220
- ecoFIT Pure 825 Repair: £120-220
- ecoFIT Pure 830 Repair: £120-220
- ecoFIT Pure 835 Repair: £120-220
- ecoTEC exclusive Repair: £120-220
- ecoTEC Plus 610 Repair: £120-220
- ecoTEC Plus 826 Repair: £120-250
- ecoTEC Plus 938 Repair: £90-180
- ecoTEC Pro Repair: £100-£180
- ecoTEC Pro 24 Repair: £100-£180
- ecoTEC Pro 28 Repair: £100-£180
- ecoTEC Pro 30 Repair: £120-200
- ecoTEC sustain Repair: £90-180
- Home Combi Repair: £120-220
- Home System Repair: £90-180
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 412 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoTEC exclusive manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026