F.786 on a Vaillant flexoTHERM exclusive: Fan unit 2: Safety cut-out open
Short answer
On a Vaillant heat pump, F.786 means: Fan unit 2: Safety cut-out open. 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: Fan unit 2: Safety cut-out open.
Part involved: Safety cut-out open.
Likely causes:
- The safety cut-out on the de-icer is open due to insufficient volume flow and/or brine temperatures above 65 °C; Operating the de-icer outside of the permitted area of application; De-icer operation when the brine circuit is not filled; During de-icer operation at brine temperatures above 115 °C, the safety fuse of the safety cut-out is tripped and the fuse must be replaced.
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: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure).
What the engineer will check
- Checking for circulation in the environment circuit pump; If required, open the stopcocks. The safety cut-out is automatically reset as soon as the temperature at the fuse falls below 30 °C again. If the safety cut-out is still open at a de-icer temperature below 65 °C (or 30 °C), temperatures above 115 °C were reached and the safety fuse tripped; Check and, if required, replace the F1 fuse of the air-to-brine collector; Replace the safety cut-out.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
What you might notice
Sources
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.54 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026