F.1017 on a Viessmann One Base: Secondary circuit flow temperature sensor downstream of condenser not recognised
Short answer
On a Viessmann heat pump, F.1017 means: Secondary circuit flow temperature sensor downstream of condenser not recognised. 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: Secondary circuit flow temperature sensor downstream of condenser not recognised.
Part involved: Secondary circuit flow temperature sensor downstream of condenser not recognised.
When it shows: System characteristics: Refrigerant circuit off.
Likely causes:
- Secondary circuit flow temperature sensor downstream of condenser not recognised.
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 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. Start the system in the following order:.
- Switch on power supply to indoor unit.
- Wait until the default display is shown on the HMI programming unit.
- Switch on power supply to outdoor unit.
- Check the sensor and connection X2.1/X2.2 on the EHCU 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. 3.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
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 161-162 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026