4A on a Viessmann Vitotronic 200: Evaporation temperature too low
Short answer
On a Viessmann heat pump, 4A means: Evaporation temperature too low. The heat pump keeps running.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Evaporation temperature too low.
Part involved: Evaporation temperature too low.
When it shows: No type given. Heat pump characteristics: Compressor remains in operation. No type given. Heat pump characteristics: Compressors remain operational.
Likely causes:
- Evaporation temperature too low.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Information only |
| Engineer needed? | No |
The heat pump keeps running.
Resetting itself: Not stated in 6150133.
Manual reset: Refrigerant circuit messages cannot be acknowledged in the message history (6150133 p.155); acknowledge the resulting 05/07/A9 message on the control unit.
What you can check
- This code is only visible to engineers in the service menu (Diagnosis > Refrigerant circuit > Message history); homeowners see message 05, 07 or A9 - note that code and call your contractor.
What the engineer will check
- Check fan.
- Check evaporator for ice formation.
- Check evaporator temperature sensor: See message code "09".
- Check primary pump.
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification.
Certification: F-gas certificate (UK F-gas Regulations; Viessmann: refrigerant work by authorised refrigeration engineers only).
What you might notice
Sources
- Service instructions Vitotronic 200 type WO1C heat pump control unit (air/water split & monobloc, brine/water) (6150133, 2023-07) ↗ · 6150133 VSA00004 · pages 158, 175, 181 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026