F.9998 on a Vaillant aroTHERM plus: Connection fault: Heat pump
Short answer
On a Vaillant heat pump, F.9998 means: Connection fault: Heat pump. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Connection fault: Heat pump.
Part involved: Heat pump.
Likely causes:
- eBUS cable not connected or connected incorrectly; Outdoor unit without supply voltage.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the heat pump supply breaker/isolator (and back-up heater breaker, if separate) at the consumer unit is on; if a breaker has tripped more than once, leave it off and call an engineer - do not open any casing. The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant.
What the engineer will check
- Check the connection pipes between the power supply PCB and control PCB on the indoor and outdoor unit.
Who to ask for: Heat pump/controls installer familiar with Vaillant eBUS controls.
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
What owners and engineers report
15 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
- Heat pump will not start on the aroTHERM plus (VWL ../6 A): 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.37; uniTOWER VIH QW 190/6 E 0020291525_05 p.64; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.61 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026