3A on a Viessmann Vitotronic 200: Safety high pressure switch has responded
Short answer
On a Viessmann heat pump, 3A means: Safety high pressure switch has responded. 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 manuals for this range word it in different ways:
- Safety high pressure switch has responded.
- Evaporator temp.sensr: No central heating or central cooling; only defrosting; The air discharge temperature sensor is used for defrosting the evaporator.
Part involved: Safety high pressure switch has responded.; Evaporator temp.sensr.
When it shows: Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor OFF Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Both compressors OFF No central heating or central cooling; only defrosting; The air discharge temperature sensor is used for defrosting the evaporator.
Likely causes:
- Safety high pressure switch has responded.
- Lead break, evaporator air intake temperature sensor.
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 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.
- Note the 2-character message code and message text shown on the Vitotronic 200 display (press OK for details) and give them to your heating contractor (operating instructions 5817913 p.56).
- You may acknowledge the message once with OK as described in the Vitotronic 200 operating instructions; if it reappears, do not keep acknowledging it - call 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 outdoor unit (air source models) is not blocked by leaves, debris or snow.
What the engineer will check
- As for "C9 Refrigerant circ (SHD)" : See "Message overview" for the heat pump control unit.
- Check resistance value Pt500A at the connection on the EEV PCB: See "EEV PCB [2] ".
- Vitocal 100-S/111-S: Check resistance value NTC 15 k Ω at the connection on the main PCB of the outdoor unit: See "Main PCB [7] / ….
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification; Viessmann-trained heat pump installer/service engineer.
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 168, 174, 163, 181, 184, 90 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026