Vaillant F.9998 fault codewhat it means and how to fix it
The heat pump has lost its eBUS connection to the controllers and wiring centre. Vaillant name an unconnected or miswired eBUS cable and an outdoor unit without supply voltage as the causes.
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What F.9998 means
F.9998 is the mirror image of the controller’s F.511. Where F.511 is the sensoCOMFORT saying it cannot hear the heat pump, F.9998 is the heat pump saying it cannot hear anything on the eBUS — Vaillant UK describe it as a communication fault between the heat pump and all eBUS controllers and wiring centres. The uniTOWER plus appendix gives two causes: the eBUS cable is not connected or is connected incorrectly, and the outdoor unit is without supply voltage. That second cause is the one an owner can do something about, because the outdoor unit has its own isolator and its own breaker, and either being off produces this code.
Vaillant’s owner-facing page goes much further and its list is worth knowing about even though every item on it is installer territory: mains voltage outside tolerance, a fault on the installer PCB, wiring faults between the heat pump and its interface, between the interface and the VR 71 wiring centre, and between the VR 71 and any wireless receivers, reversed polarity on the eBUS at any terminal, cable of insufficient diameter — Vaillant specify a minimum of 0.75 mm² twin core — induced current from mains cables run closer than 120 mm, the wrong cable type, since twisted multicore is not permissible, shielded cable that has not been suitably earthed, and finally a faulty HMU.
Read as a whole, that list says F.9998 is very often an installation-quality fault rather than a failed component, which is why it tends to appear on newly commissioned systems, after an electrician has been in, or when a wiring centre has been disturbed.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Outdoor unit without supply voltage | Vaillant’s published cause and the one to check first: an isolator switched off, a breaker tripped, or a power cut that has not restored both units. | Common |
| eBUS cable not connected or connected incorrectly | Vaillant’s other published cause. Reversed polarity at any terminal, a loose core, or a connection disturbed during other work. | Common |
| Wrong cable or a poor route | Vaillant rule out twisted multicore, require at least 0.75 mm² twin core, and want 120 mm separation from mains cables. Installations that break these rules produce intermittent F.9998. | Occasional |
| Installer PCB or HMU fault | The component end of Vaillant’s list, reached after the wiring and the supply have been proven. | Occasional |
What to try yourself
Check the outdoor unit has power. There is normally a rotary isolator on the wall near it and a dedicated breaker in the consumer unit; either being off is one of Vaillant’s two published causes.
Check the indoor unit and the wiring centre — usually a VR 71 box near the cylinder — are also powered and have not been switched off during other work.
Think about recent visits. An electrician, a new socket, a smart thermostat installation or anyone working in the cupboard can disturb an eBUS terminal.
Ring your installer with the code. Everything else on Vaillant’s list — polarity, cable type, separation from mains, the installer PCB — is theirs to check.
Do not open the wiring centre or either unit to inspect eBUS terminals. Mains terminals sit in the same enclosures.
Get an F-Gas certified heat pump engineer to look at your Vaillant F.9998
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£90–£300
Time on site
15 minutes
Nothing if an isolator was off. An installer visit to trace the eBUS fault is typically £90 to £180, and rerunning a badly routed cable is a few hours’ labour. An installer PCB is the expensive outcome.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Vaillant fault codes
- F.511Get it checkedThe sensoCOMFORT controller has lost contact with the heat pump control module. Vaillant give two causes, a defective cable or a plug connection, and the house may stay warm while it shows.Related: controller and communication
- Radio communication interruptedGet it checkedA message from the wireless sensoCOMFORT VRC 720f, not from the heat pump. Vaillant’s first instruction is to make sure the power supply is on; the second concerns reception strength.Related: controller and communication
- E5Usually simple to fixE5 is not a Vaillant heat pump code. Vaillant’s aroTHERM, uniTOWER and sensoCOMFORT fault tables run F.5xx to F.9998, so an E5 on your display belongs to something else in the system.Related: controller and communication
- F.22Get it checkedWater pressure too low. Vaillant publish exact thresholds and two instructions: the indoor unit manual says inform your competent person; Vaillant UK’s fault page lets a competent owner top up to 1 to 1.5 bar.Related: same brand
- Vapour from the outdoor unitUsually simple to fixA cloud rising off the outdoor unit is the defrost cycle. Vaillant’s own wording is that you do not have to do anything.Related: same brand
- Running all the timeUsually simple to fixA heat pump is meant to run long and low. The alarm signal is not the running time, it is the bill — and turning the thermostat up will not warm the house faster.Related: same brand
Own a Vaillant heat pump? Save it and its codes come first next time.
Related guides
Common questions about Vaillant F.9998
What is the difference between F.9998 and F.511?
Direction. F.511 is shown by the sensoCOMFORT controller when it cannot reach the heat pump control module. F.9998 is shown by the heat pump when it has lost the eBUS to all its controllers. Both can be caused by a unit without power, and both are wiring faults once power is proven.
It appeared straight after installation. Is that normal?
It is common, and Vaillant’s cause list explains why: cable diameter, cable type, polarity, separation from mains and earthing of shielded cable are all commissioning details. A new system showing F.9998 should go straight back to the installer as a snag rather than a repair.
Will the heat pump run without the eBUS?
Not usefully. The heat pump takes its demand from the controller over the eBUS, so with the link down it has nothing to respond to. The back-up heater may be called instead depending on how the system was configured, and that shows up on the electricity bill.
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
- Manufacturer support portal
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.