BoilerFaultCodesVaillant F.84 Fault Code
Also written as F84 — same fault.
The Vaillant F.84 fault code means: your boiler is confused because its internal thermometers are giving conflicting readings and don't agree on how hot the water actually is.. It appears on 18 Vaillant models in our database.
Technical description: Fault: Flow/return temperature sensor Values not consistent, difference between flow and return <-6K Flow and return temperature sensors signalling implausible values: Flow and return temperature sensors have been inverted, flow and return temperature sensors have not been correctly installed
What causes the Vaillant F.84 fault?
This fault occurs when the boiler's internal sensors provide temperature readings that are too far apart for the system to trust, suggesting the water is heating up in a way that doesn't make sense. The most common reason is that one of the sensors has developed a chemical or Electrical fault, but it can also happen if the sensors are incorrectly installed or have become loose from the internal pipework.
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.84 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £120-220
- ecoFIT Pure 418 Repair: £150-250
- ecoFIT Pure 425 Repair: £150-250
- ecoFIT Pure 435 Repair: £150-250
- ecoFIT Pure 825 Repair: £150-250
- ecoFIT Pure 830 Repair: £150-250
- ecoFIT Pure 835 Repair: £150-250
- ecoTEC exclusive Repair: £120-250
- ecoTEC Plus 610 Repair: £150-£250
- ecoTEC Plus 826 Repair: £150-300
- ecoTEC Plus 938 Repair: £120-£250
- ecoTEC Pro Repair: £100-£180
- ecoTEC Pro 24 Repair: £100-£180
- ecoTEC Pro 28 Repair: £100-£180
- ecoTEC Pro 30 Repair: £150-250
- ecoTEC sustain Repair: £120-£250
- Home Combi Repair: £150-250
- Home System Repair: £120-220
Source
Vaillant F.84 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 exclusive manual (PDF)
- Vaillant ecoTEC Plus 610 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026