F.1174 on a Viessmann One Base: Forced safety shutdown, compressor - interlock input 2 active
Short answer
On a Viessmann heat pump, F.1174 means: Forced safety shutdown, compressor - interlock input 2 active. 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: Forced safety shutdown, compressor - interlock input 2 active.
Part involved: Forced safety shutdown, compressor - interlock input 2 active.
When it shows: System characteristics:; Output demand to heat pump not possible; Compressor not available.
Likely causes:
- Forced safety shutdown of the compressor.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| 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: 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.
- 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 inverter safety circuit for correct function.
- Disconnect connection X3.1/X3.2 (STO2) from the inverter. Check for continuity. If necessary, replace high limit safety cut-out.
- Replace inverter.
- Unlock refrigerant circuit.
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 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 187 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026