F.996 on a Viessmann One Base: Short circuit, temperature sensor in cooling water buffer cylinder
Short answer
On a Viessmann heat pump, F.996 means: Short circuit, temperature sensor in cooling water buffer cylinder. 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: Short circuit, temperature sensor in cooling water buffer cylinder.
Part involved: Short circuit, temperature sensor in cooling water buffer cylinder.
When it shows: System characteristics: No cooling of external cooling water buffer cylinder.
Likely causes:
- Only for heat pumps with 1 integral heating/cooling circuit: Short circuit, temperature sensor in cooling water buffer cylinder.
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 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).
- Check the operating program, time program and room temperature settings on the HMI.
What the engineer will check
- Check the sensor and the following connections:.
- Terminals X17.1/X17.2 on EHCU electronics module.
- Wall mounted indoor unit: 6-pole connection socket on underside of appliance on left, terminals 5 and 6.
- Floorstanding indoor unit: Terminal area for extra low voltage (ELV) < 42 V, luster terminal for sensors, terminals 7 and 8.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
- Check resistor R for NTC 10 kΩ on the disconnected plug. If R < 500 Ω, replace the sensor.
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 174-175 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026