F.685 on a Viessmann One Base: Communication error, HPMU electronics module
Short answer
On a Viessmann heat pump, F.685 means: Communication error, HPMU electronics module. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Communication error, HPMU electronics module.
Part involved: Communication error, HPMU electronics module.
When it shows: System characteristics:; Function of connected components in emergency mode; Frost protection enabled; Passive frost protection on EHCU electronics module.
Likely causes:
- HPMU electronics module communication error.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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 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 system from the power supply. Wait at least 2 min. Switch on the system.
- Check the CAN bus connection between the indoor unit and outdoor unit (CAN bus communication cable):.
- Wall mounted indoor unit: Connection on the underside of the appliance, 5pin connection socket on the right, terminals 72.L to 72.H.
- Indoor unit with integral DHW cylinder/loading cylinder: Terminal area for extra low voltage (ELV) < 42 V, luster terminal for bus connections, terminals 72.3 to ….
- 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).
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 124 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026