N.521 on a Glow-worm GeniaAir: Outdoor temperature sensor signal invalid
Short answer
On a Glow-worm heat pump, N.521 means: Outdoor temperature sensor signal invalid. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Outdoor temperature sensor signal invalid.
Part involved: Outdoor temperature sensor signal invalid.
Likely causes:
- Outdoor temperature sensor not connected, defective or not installed.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no - irreversible; requires action.
Manual reset: engineer rectifies the cause.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). 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 settings on the control; check the outdoor temperature sensor; if no sensor is installed deactivate weather compensation via D.162.
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