F.1211 on a Viessmann One Base: CAN bus communication fault between indoor and outdoor unit
Short answer
On a Viessmann heat pump, F.1211 means: CAN bus communication fault between indoor and outdoor unit. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: CAN bus communication fault between indoor and outdoor unit.
Part involved: CAN bus communication fault between indoor and outdoor unit.
When it shows: System characteristics: The refrigerant circuit will not start.
Likely causes:
- Communication fault with the VCMU refrigerant circuit controller.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
Resetting itself: Not stated per code in Viessmann document 6199893 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.
What the engineer will check
- Only Vitocal 250-A.../252-A, sizes 16 to 19:.
- Check the CAN bus connection between the indoor unit and outdoor unit:.
- Wall mounted indoor unit: Connection on underside of appliance, 5-pin connection socket on the right, terminals 72.L to 72.H Floorstanding indoor unit: Terminal area for ….
- Connection between X7.1 to X7.3 on the VCMU refrigerant circuit controller and X5.1 to X5.3 on the EHCU electronics module.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
- Shielding must be connected to CAN Ground (GND).
Who to ask for: Viessmann-trained heat pump engineer with F-gas certification and R290 (A3 flammable refrigerant) competence.
Certification: F-gas certificate (UK F-gas Regulations; EU 2015/2067 Cat. I cited by Viessmann) plus A3/R290 competence per EN 378-4 or IEC 60335-2-40 Annex HH (6202263 p.2).
Sources
- Service instructions for contractors: Heat pumps in monoblock design incl. hybrid – System configuration and diagnostics (Vitocal 150-A/151-A/250-A/252-A, IDU-A, AWMOF-151/251, AquaSnap 30AWH, BI-BLOC R290) (6199893 GB 01/2026) ↗ · 6199893 GB 01/2026 (VSA00009) · page 191-192 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026