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 21 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.
- ecoFIT Pure 435 Repair: £100-250
- ecoFIT pure 612 Repair: £150-250
- ecoFIT pure 615 Repair: £150-250
- ecoFIT pure 618 Repair: £150-250
- ecoFIT pure 630 Repair: £150-250
- ecoFIT Pure 825 Repair: £150-250
- ecoFIT Pure 830 Repair: £150-250
- ecoFIT Pure 835 Repair: £150-250
- ecoFIT sustain 615 Repair: £150-250
- ecoFIT sustain 618 Repair: £150-250
- ecoFIT sustain 630 Repair: £150-250
- 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: £120-£250
- Home Combi 25 Repair: £150-250
- Home Combi 30 Repair: £150-250
- Home Combi 35 Repair: £150-250
- Home Regular 12
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 435 manual (PDF)
- Vaillant ecoFIT pure 630 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026