F.1320 on a Viessmann One Base: Communication error between HPMU and EHCU
Short answer
On a Viessmann heat pump, F.1320 means: Communication error between HPMU and EHCU. 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 error between HPMU and EHCU.
Part involved: Communication error between HPMU and EHCU.
When it shows: System characteristics: Limited normal operation Limited control of secondary pump/heating circuit pump for heating/cooling circuit 1.
Likely causes:
- Communication error between the HPMU and EHCU electronics modules Lead break, PWM feedback signal, secondary pump/ heating circuit pump for heating/cooling circuit 1.
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
- Check the following CAN bus connections:.
- 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 extra low voltage (ELV) < 42 V, luster terminal for bus connections, terminals 72.3 to 72.5.
- Connection between X10.1 to X10.5 on the EHCU electronics module and X4.1 to X4.5 on the HPMU 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.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
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 198-199 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026