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 19 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.
- 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: £120-250
- ecoFIT Pure 835 Repair: £120-200
- ecoFIT sustain 615 Repair: £100-£200
- ecoFIT sustain 618 Repair: £100-£200
- ecoFIT sustain 630 Repair: £100-£200
- ecoFIT sustain 825 Repair: £90-180
- ecoFIT sustain 830 Repair: £90-180
- ecoFIT sustain 835 Repair: £90-180
- ecoTEC plus Repair: £80-180
- ecoTEC pro Repair: £90-180
- Home Combi 25 Repair: £120-250
- Home Combi 30 Repair: £120-250
- Home Combi 35 Repair: £120-250
- Home Regular 12
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 630 manual (PDF)
- Vaillant ecoFIT Pure 835 manual (PDF)
- Vaillant ecoFIT sustain 630 manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026