Vaillant F9998 is a communication error. If you have a 2026 aroTHERM Pro or Plus, the indoor and outdoor units need to be turned on at the same time.
I have asked Vaillant about this, and they have confirmed that there is no way around the simultaneous power-on for these models. Turning one on, walking outside and turning the other on is not the same thing.
On older models, or if the fault keeps returning, I would look at the power supply, loose connections, a broken wire and the route of the communication cable.
What does Vaillant F9998 mean?
The display may show F.9998 or F9998. Vaillant's appliance-interface manual calls it Connection fault: Heat pump
. The heat pump and its controls are not communicating properly.
That does not automatically mean a circuit board needs replacing. A unit without power cannot communicate, and neither can a unit with a poor cable connection.
- 2026 Pro or Plus after switching the power on: start with the simultaneous power-on below.
- New installation: a loose connection is the first wiring fault I would suspect.
- Fault coming and going: check connections and whether the bus cable runs too close to mains wiring.
Fault definition and published causes: Vaillant UK heat-pump fault codes. The 2026 restart advice here comes from my conversation with Vaillant.
F9998 on the 2026 aroTHERM Pro and Plus
The important part is restoring power to the outdoor heat pump and the indoor unit or wiring centre together. The room controller's heating-off setting is not the same as turning off the power supply.
- Photograph the fault. Note whether it appeared after a power cut, electrical work or turning a unit off.
- Stop the heat pump normally. Follow its operating instructions, including any shutdown or restart waiting time.
- Switch off both relevant supplies. Identify the outdoor unit and the indoor unit or wiring-centre supply. Do not guess from an unlabelled breaker.
- Restore both supplies at the same time. Choose an arrangement below that suits the installation.
- Allow the system to finish starting. Check that communication returns and that heating or hot water operates when there is demand. If F9998 remains or returns, arrange a fault investigation.
This is a restart, not safe isolation for electrical work. Switching the main switch off does not make every part inside electrical equipment safe to touch. Do not leave the heat pump without power in freezing weather; contact your installer promptly if it cannot be restored. Where Flexible Space Function is used, its protection also relies on the outdoor unit remaining powered.
Older Vaillant heat pumps and new-installation wiring faults
If it is an older model, look for a loose connection or a broken wire. On a new installation, a loose connection is probably the most likely wiring problem.
These are engineer checks. The indoor unit showing a display does not prove that the outdoor unit has its supply. An engineer needs to check both supplies, the communication cable and its connections before condemning a board.
- Check the cable is connected to the correct terminals for the installed equipment.
- Check for a loose terminal, a poorly seated plug or a damaged conductor.
- Check the cable specification and route, not just its ends.
- If those checks do not find the cause, follow Vaillant's fault-finding procedure for the interface, internal connections and boards.
All of that requires the appropriate safe-isolation and testing procedure. It is not a reason for a homeowner to open the wiring centre.
The Pro and Plus comparison explains the current model codes. The date a heat pump was installed is not necessarily its model generation.
Intermittent F9998: check the bus cable route
Vaillant has advised me to keep the bus cable at least 150mm away from mains power cables. If it has been run alongside mains wiring, interference can cause strange communication errors, and F9998 is one of them.
I have never encountered this problem myself. I am always careful with cable separation because I do not want callbacks.
This is worth checking on both newer and older installations. A cable can be securely connected at each end and still have a routing problem between them.
Vaillant's published F.9998 guidance specifies a minimum separation of 120mm. The 150mm advice I have been given also comes from Vaillant. Follow the cable-type and routing requirements for the particular equipment, including any stated exceptions at enclosures or wall penetrations.
Tell the engineer if the fault coincides with other equipment switching on, or started after electrical work. That is useful evidence, not proof of the cause on its own.
What to record if F9998 does not clear
Do not keep switching the system off and on. Send your installer:
- A photograph of the full fault message.
- The outdoor model number and indoor unit or wiring-centre model, where visible without removing covers.
- Whether the fault is permanent or intermittent.
- What happened immediately before it appeared.
- Whether the simultaneous restart was tried and what happened afterwards.
For local help, our heat-pump servicing team works across Lincoln and the surrounding area. Tell us it is a fault investigation when booking, so we allow time to find the cause.
Vaillant F9998 questions
What does Vaillant F9998 mean?
F9998, also written F.9998, is a communication fault between the heat pump and its controls or wiring centre. Missing outdoor power, a wiring fault or interference on the communication cable can cause it.
How do I restart a 2026 aroTHERM Pro or Plus with F9998?
The indoor and outdoor units need to be powered on at the same time. This is the advice Vaillant gave me. Use the identified external switches, following the safety checks above. If the fault remains or returns, get the installation checked.
Can I just restart the outdoor unit?
For the 2026 Pro and Plus fault described here, restarting the outdoor unit on its own is not the procedure. Both the indoor and outdoor supplies need to come on together.
Why does F9998 keep coming back?
Check the wiring and power supply rather than repeatedly resetting it. Loose connections, a damaged cable and interference where the bus cable runs alongside mains wiring are things to investigate.
How far should the bus cable be from mains cables?
Vaillant has advised me to keep the bus cable at least 150mm away from mains power cables. Their published F.9998 guidance lists 120mm; the 150mm advice also comes from Vaillant, not a figure I have chosen myself.
Is F9998 a Wi-Fi fault?
F9998 concerns communication within the heating system. It is not simply the myVAILLANT app losing its internet connection, so restarting the router is not the starting point.
Technical references
- Vaillant VWZ AI manual: F.9998 fault entry, page 37.
- Vaillant sensoCOMFORT manual: eBUS cable requirements, page 22.
- Electrical Safety First: safe-isolation guidance for electricians.
Use the instructions for the actual units and controls fitted. The simultaneous-start procedure is my practical advice following discussion with Vaillant; it is not a quotation from these manuals.
