F.1259 on a Viessmann One Base: Modbus communication fault, refrigerant circuit controller (VCMU) to inverter
Short answer
On a Viessmann heat pump, F.1259 means: Modbus communication fault, refrigerant circuit controller (VCMU) to inverter. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Modbus communication fault, refrigerant circuit controller (VCMU) to inverter.
Part involved: Modbus communication fault, refrigerant circuit controller (VCMU) to inverter.
When it shows: System characteristics: Refrigerant circuit off.
Likely causes:
- No communication between inverter and heat pump control unit.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
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.
- Check the heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).
What the engineer will check
- Isolate the outdoor unit from the power supply. Wait at least 2 min. Switch on voltage again.
- Check the following connections for the Modbus:.
- Only Vitocal 250-A.../252-A, sizes 16 to 19: From connection X11.1 to X11.3 on the VCMU refrigerant circuit controller to connection IPS_X1.5 to IPS_X1.7 on the ….
- Other heat pumps: From connection X11.1 to X11.3 on the VCMU refrigerant circuit controller to connection I1.5 to I1.7 on the inverter.
- 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). Replace Modbus cable if necessary.
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).
What you might notice
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 197 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026