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 43 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.
Find a Gas Safe engineer
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A cover plan would handle faults like F.78: no call-out fees, no surprise bills.
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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 435 Repair: £120-200
- ecoFIT pure 612
- ecoFIT pure 615
- ecoFIT pure 618
- ecoFIT pure 630
- ecoFIT Pure 825 Repair: £150-250
- ecoFIT Pure 830 Repair: £120-250
- ecoFIT Pure 835 Repair: £120-200
- ecoFIT sustain 615
- ecoFIT sustain 618
- ecoFIT sustain 630
- ecoFIT sustain 825
- ecoFIT sustain 830
- ecoFIT sustain 835
- ecoTEC Repair: £80-180
- ecoTEC exclusive 627
- ecoTEC plus 412 Repair: £120-250
- ecoTEC plus 415 Repair: £120-250
- ecoTEC plus 418 Repair: £120-250
- ecoTEC plus 424 Repair: £120-250
- ecoTEC plus 430 Repair: £120-250
- ecoTEC Plus 610 Repair: £120-250
- ecoTEC plus 612
- ecoTEC Plus 615 Repair: £90-180
- ecoTEC plus 618
- ecoTEC Plus 620 Repair: £90-180
- ecoTEC plus 624
- ecoTEC Plus 625 Repair: £90-180
- ecoTEC Plus 630 Repair: £90-180
- ecoTEC Plus 635 Repair: £90-180
- ecoTEC plus 637 Repair: £120-250
- ecoTEC Plus 826 Repair: £120-250
- ecoTEC Plus 832 Repair: £90-180
- 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 ecoFIT pure 630 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026