BoilerFaultCodesVaillant F.84 Fault Code
Also written as F84 — same fault.
The Vaillant F.84 fault code means: your boiler has noticed that its two internal temperature sensors are giving conflicting readings, suggesting they are either broken or have been installed incorrectly.. It appears on 43 Vaillant models in our database.
Technical description: Error with flow/return temperature sensor temperature changing error
What causes the Vaillant F.84 fault?
This fault occurs when the boiler's control board detects that the flow and return temperature sensors are reporting contradictory data, such as the return pipe being hotter than the flow pipe. The most common reason is that the sensors have been clipped onto the wrong pipes during a repair, or the internal sensors have developed a chemical fault and are sending incorrect electrical signals.
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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A cover plan would handle faults like F.84: no call-out fees, no surprise bills.
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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 435 Repair: £150-250
- ecoFIT pure 612
- ecoFIT pure 615
- ecoFIT pure 618
- ecoFIT pure 630
- ecoFIT Pure 825 Repair: £150-250
- ecoFIT Pure 830 Repair: £150-250
- ecoFIT Pure 835 Repair: £150-250
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 825
- ecoFIT sustain 830
- ecoFIT sustain 835
- ecoTEC
- ecoTEC exclusive 627
- ecoTEC plus 412 Repair: £150-£250
- ecoTEC plus 415 Repair: £150-£250
- ecoTEC plus 418 Repair: £150-£250
- ecoTEC plus 424 Repair: £150-£250
- ecoTEC plus 430 Repair: £150-£250
- ecoTEC Plus 610 Repair: £150-£250
- ecoTEC plus 612
- ecoTEC plus 618
- ecoTEC Plus 620 Repair: £120-£250
- ecoTEC plus 624
- ecoTEC Plus 625 Repair: £120-£250
- ecoTEC Plus 630 Repair: £120-£250
- ecoTEC Plus 635 Repair: £120-£250
- ecoTEC plus 637
- ecoTEC Plus 826 Repair: £150-300
- ecoTEC Plus 832 Repair: £120-£250
- ecoTEC Plus 840 Repair: £120-£250
- ecoTEC Plus 938 Repair: £120-£250
- ecoTEC Pro Repair: £100-£180
- ecoTEC Pro / ecoTEC Plus Repair: £90-180
- 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 ecoFIT pure 630 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026