F.900 on a Vaillant aroTHERM plus: Internet module communication interrupted
Short answer
On a Vaillant heat pump, F.900 means: Internet module communication interrupted. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Internet module communication interrupted.
Part involved: sensoCOMFORT system control / eBUS modules (FM3 = VR 70, FM5 = VR 71).
Likely causes:
- Cable defective; incorrect plug connection (internet gateway, e.g. sensoNET/myVAILLANT connect).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
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 cable; check the plug connection.
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
11 owner and engineer reports mention this.
- App / connectivity problem on the aroTHERM plus (VWL ../6 A): 11 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