F.1034 on a Viessmann One Base: Communication fault, external CAN bus
Short answer
On a Viessmann heat pump, F.1034 means: Communication fault, external CAN bus. 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: Communication fault, external CAN bus.
Part involved: Communication fault, external CAN bus.
When it shows: System characteristics: Limited normal operation.
Likely causes:
- Short circuit, external CAN bus.
- Connections 91.CAN L/91.CAN H interchanged.
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 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
- Turn off the ON/OFF switch on the indoor unit. Wait at least 2 min. Turn on the ON/OFF switch.
- Check configuration for correct HPMU electronics module.
- Check the following CAN bus connections:.
- Connection X8.1 to X8.3 on plug 91 on HPMU electronics module.
- Wall mounted indoor unit: 6-pole connection socket on underside of appliance on left, terminals 91.CAN L to 91.CAN H Floorstanding indoor unit: Terminal area for ….
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
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 179 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026