F.984 on a Viessmann One Base: Repeated control fault, electronic expansion valve 1
Short answer
On a Viessmann heat pump, F.984 means: Repeated control fault, electronic expansion valve 1. 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: Repeated control fault, electronic expansion valve 1.
Part involved: Repeated control fault, electronic expansion valve 1.
When it shows: System characteristics: The refrigerant circuit will not start.
Likely causes:
- Control fault, electronic expansion valve 1:.
- Stator of expansion valve 1 faulty.
- Expansion valve 1 blocked in "open" position.
- Expansion valve 1 clogged.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Severity | Safety lockout |
| 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: Measures include resetting/unlocking the refrigerant circuit via HMI service menu "Reset outdoor unit" or ViGuide (engineer, password-protected; 6199893 p.118).
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 connections X8.1 to X8.5 on the VCMU refrigerant circuit controller:.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage. Replace cable if necessary.
- Isolate the system from the power supply. Wait at least 2 min. Switch on the system.
- Check electronic expansion valve 1 via actuator test.
- Check resistor R for 20 °C at the disconnected plug of electronic expansion valve 1: On all 4 coils of the stator between pin 5 ….
- Replace the VCMU refrigerant circuit controller.
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 171 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026