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 34 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
Search the official register for a qualified engineer in your area.
Gas Safe Register →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
- Combi Compact 242EB Repair: £120-220
- Combi Compact 282EB Repair: £120-220
- ecoFIT pure Repair: £150-250
- ecoFIT Pure 412 Repair: £150-250
- 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 Repair: £120-£250
- ecoTEC Plus 610 Repair: £150-£250
- ecoTEC Plus 615 Repair: £120-£250
- ecoTEC Plus 620 Repair: £120-£250
- ecoTEC Plus 625 Repair: £120-£250
- ecoTEC Plus 630 Repair: £120-£250
- ecoTEC Plus 635 Repair: £120-£250
- ecoTEC Plus 65 Repair: £120-£250
- ecoTEC Plus 826 Repair: £150-300
- ecoTEC Plus 832 Repair: £120-£250
- ecoTEC Plus 836 Repair: £120-£250
- ecoTEC Plus 840 Repair: £120-£250
- ecoTEC Plus 938 Repair: £120-£250
- ecoTEC plus Regular Repair: £150-250
- ecoTEC plus System Repair: £150-250
- 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 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.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026