Vaillant F.78 fault codewhat it means and how to fix it
A wiring-configuration code on UK combis: the external control's link box is connected but the hot water sensor input has not been bridged. Vaillant say the boiler itself has not malfunctioned.
Take care
Get a Gas Safe registered engineer to look at your Vaillant F.78
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What F.78 means
F.78 is peculiar to the UK market and to boilers wired to certain external controls through Vaillant's link box. The ecoTEC plus and pro tables print it as "interruption: DHW outlet sensor at external control — UK link box is connected without the domestic hot water temperature sensor being bridged", tell the engineer to check the accessory's configuration and electrical connection, and then add a line no other row carries: "the unit displays a fault, but the unit has not malfunctioned".
The exclusive and ecoFIT pure repeat the same cause in fewer words. In other words, nothing inside the boiler is broken; a link that the wiring instructions require at the accessory has been left out, and the board reads the missing link as an open sensor. The earlier ecoTEC servicing instructions of 2005 to 2012 show the same code from the other end — "wrong configuration with accessory: link box VR65 connected to combination boiler" — because the VR 65 was meant for system boilers and a combi fitted with one produces F.78.
On the 2022 CS/1-5 the number means something else again: F.078 is "the control centre is not supported by the unit — incorrect control module connected", a compatibility check on the control rather than a missing bridge. Every version is a job for whoever wired the control, and none is a reason to suspect the boiler.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Link box wired without the hot water sensor bridge | Vaillant's stated cause on the ecoTEC plus, pro, exclusive and ecoFIT pure. The UK link box needs the DHW sensor terminals bridged on a combi and the installer has left them open. | Common |
| VR 65 link box fitted to a combi | The 2005 to 2012 meaning: an accessory intended for a system boiler connected to a combination boiler, which the board reads as a wrong configuration. | Occasional |
| Control not supported by a 2022 CS/1-5 | F.078 on the three-digit display is a compatibility fault. A control module the boiler does not recognise has been connected, and Vaillant's check is whether it is one recommended for the product. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£80–£120
Time on site
30 minutes
Correcting the link box wiring is a call-out of £80 to £120 if the original installer will not return. No boiler parts are involved.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Vaillant wiring harness (plug-and-cable set to a sensor or actuator)
£20–£65Authorised Vaillant parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.49Get it checkedThe voltage on the eBUS, the two-wire link between the boiler and its Vaillant controls, has dropped too low. Usually a short or a wiring clash on that bus, not a boiler fault.Related: controls and communication
- conGet it checkedThe display has lost contact with the main board. "con" on a Vaillant is a communication fault between the user interface and the PCB in the electronics box, not a fault in the heating itself.Related: controls and communication
- S.39Get it checkedA protective wait, not a lockout: the "burner off" contact has opened — an underfloor thermostat or a condensate pump alarm wired to the boiler — and heating is blocked until it closes again.Related: controls and communication
- S.42Get it checkedA protective wait, not a lockout: an external device — a flue non-return flap through the VR40 module, or a condensate pump — is telling the boiler not to fire. Heating waits until it clears.Related: controls and communication
- F.90Get it checkedCommunication with the actoSTOR hot water module — or on the 2022 CS/1-5 the internal cylinder — has been interrupted. A wiring fault on the link, or a setting left on after the module went.Related: controls and communication
- F.86Get it checkedAn ecoFIT code: the underfloor heating contact is open. A floor-protection thermostat wired to the boiler has tripped, or its sensor is disconnected, and heating is blocked until it closes.Related: controls and communication
Own a Vaillant boiler? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.78
Vaillant say the boiler has not malfunctioned, so why has it stopped?
Because the board treats the missing bridge as an open sensor and will not run on it. The fix is in the accessory wiring, which is why Vaillant route it to the installer: the boiler will behave normally the moment the link is made.
Who is responsible for fixing it?
The firm that fitted or rewired the control. F.78 appears when the link box is connected wrongly, so it normally shows up straight after that work and is theirs to correct.
I have a 2022 boiler with a three-digit display showing F.078. Same fault?
No. On the CS/1-5 the number means the connected control is not one the boiler supports. It is still a controls question rather than a boiler fault, but the remedy is a compatible control, not a bridge.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.