F.521 on a Vaillant aroTHERM plus: Outdoor temperature sensor signal invalid
Short answer
On a Vaillant heat pump, F.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: sensoCOMFORT system control / eBUS modules (FM3 = VR 70, FM5 = VR 71).
Likely causes:
- Outdoor temperature sensor defective.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - message clears when communication/configuration is restored.
Manual reset: not normally required; configuration corrected by installer.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the system control display is lit; if it is dark or frozen, hold the top-right button for more than 5 seconds to restart it, or switch the heat generator's mains switch off for about 1 minute and back on (sensoCOMFORT operating instructions). If the message persists, inform your competent person.
What the engineer will check
- Replace the outdoor temperature sensor.
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
5 owner and engineer reports mention this.
- Insufficient heating (house not warm enough) on the aroTHERM plus (VWL ../6 A): 3 reports
- Heating on when not wanted / house too hot on the aroTHERM plus (VWL ../6 A): 2 reports
Sources
- sensoCOMFORT VRC 720/3 operating and installation instructions ↗ · 0020334316_03 · page sensoCOMFORT VRC 720/3 0020334316_03 p.55 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026