F.9998 on a Glow-worm GeniaAir: No communication is possible between the indoor unit and the outdoor unit
Short answer
On a Glow-worm heat pump, F.9998 means: No communication is possible between the indoor unit and the outdoor unit. 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: No communication is possible between the indoor unit and the outdoor unit.
Part involved: No communication is possible between the indoor unit and the outdoor unit.
Likely causes:
- Communication cable not connected or incorrectly connected. Outdoor unit without supply voltage.
- Glow-worm trade page: Communication fault between the heat pump to all Ebus controllers/wiring centres Incoming main voltage is not within permissible tolerances to heat pump, fault in the wiring or connection between the heat pump interface and VR71, fault in the wiring or connection between the VR71 and ….
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. Glow-worm's MiPro Sense operating instructions say always contact your competent person in the event of a fault message; until they arrive you may select a limp-home (back-up heater) setting on the control, which is expensive to run.
What the engineer will check
- Check the communication cables 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
Sources
- GeniaAir Appliance Interface installation and maintenance instructions ↗ · 8000043014_01 · page HPIM 7 8000043014_01 p.35 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026