L.788 on a Glow-worm GeniaAir: The building circuit pump reports an internal fault. The unit attempts to restart
Short answer
On a Glow-worm heat pump, L.788 means: The building circuit pump reports an internal fault. The unit attempts to restart. The heat pump keeps running with some functions stopped.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: The building circuit pump reports an internal fault. The unit attempts to restart.
Part involved: The building circuit pump reports an internal fault. The unit attempts to restar.
Likely causes:
- The building circuit pump reports an internal fault. The unit attempts to restart.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - reversible L-codes eliminate themselves; the unit attempts to restart.
Manual reset: not required.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). The unit restarts by itself; if the code keeps appearing, or an F-code follows, contact your installer.
What the engineer will check
- Monitor; if it recurs or turns into the corresponding F-code, diagnose as for that F-code. Corresponding lockout fault: F.788.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
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.30 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026