BoilerFaultCodesVaillant F.78 Fault Code
Also written as F78 — same fault.
The Vaillant F.78 fault code means: your boiler is confused because it is trying to communicate with an external control box that isn't compatible with its current setup.. It appears on 25 Vaillant models in our database.
Technical description: wrong configuration with accessory | link box VR65 connected to combination boiler
What causes the Vaillant F.78 fault?
This fault occurs when the boiler's main control board detects an external wiring box that was designed for older hot water systems but is not compatible with your specific boiler model. It essentially creates a digital 'clash' where the boiler receives signals it doesn't understand, leading it to shut down for safety. The most common reason is that an incorrect accessory was installed or the boiler's internal software has been configured for a system type that doesn't match the physical parts attached.
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.78 fault code
Click a model for the full fault code list and model-specific repair notes.
- Atmomax Repair: £90-180
- ecoFIT Pure 418 Repair: £120-200
- ecoFIT Pure 425 Repair: £120-200
- ecoFIT Pure 435 Repair: £120-200
- ecoFIT Pure 825 Repair: £150-250
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835 Repair: £120-200
- ecoTEC Repair: £80-180
- ecoTEC exclusive Repair: £120-250
- ecoTEC Plus 610 Repair: £120-250
- ecoTEC Plus 615 Repair: £90-180
- ecoTEC Plus 620 Repair: £90-180
- ecoTEC Plus 625 Repair: £90-180
- ecoTEC Plus 630 Repair: £90-180
- ecoTEC Plus 635 Repair: £90-180
- ecoTEC Plus 826 Repair: £120-250
- ecoTEC Plus 840 Repair: £90-180
- ecoTEC Plus 938 Repair: £90-180
- ecoTEC Pro Repair: £90-180
- ecoTEC Pro / ecoTEC Plus Repair: £120-250
- ecoTEC Pro 24 Repair: £90-180
- ecoTEC Pro 28 Repair: £90-180
- ecoTEC Pro 30 Repair: £120-250
- Home Combi Repair: £120-250
- Home System Repair: £90-180
Source
Vaillant F.78 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 manual (PDF)
- Vaillant ecoTEC exclusive manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026