F.520 on a Viessmann One Base: Internal fault, gateway
Short answer
On a Viessmann heat pump, F.520 means: Internal fault, gateway. 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: Internal fault, gateway.
Part involved: Internal fault, gateway.
When it shows: System characteristics:; Normal operation; No external demand possible.
Likely causes:
- Internal fault on gateway (MB/RTU or MB/TCP).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: Not stated per code in Viessmann document 6199961 GB 01/2026 (VSA00009).
Manual reset: User may acknowledge via HMI after noting the code (6249162 p.57); the message returns if the cause persists.
What you can check
- On the HMI, tap the message and "?" to read the system characteristics and Viessmann's user tips; note the code (e.g. F.xxx) and text for your contractor (operating instructions 6249162 p.57).
- Acknowledge the message once via the HMI; do not acknowledge fault messages several times in quick succession (6249162 p.58) - notify your contractor.
- Check your home router/internet connection is working.
What the engineer will check
- Check system configuration for gateway: CAN bus subscriber number 90.
- Turn off the ON/OFF switch on the indoor unit. Wait at least 2 min. Turn on the ON/OFF switch.
- Replace gateway.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
Sources
- Service instructions: Heat pumps in split design incl. hybrid – System configuration and diagnostics (Vitocal 200-S/222-S One Base) (6199961 GB 01/2026) ↗ · 6199961 VSA00009 · page 119 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026