18 on a Viessmann Vitotronic 200: Compressor fault (general message)
Short answer
On a Viessmann heat pump, 18 means: Compressor fault (general message). The manual does not say whether the heat pump stops. 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:
- Compressor fault (general message).
- Vitocal 300-G/333-G: Compressor fault (general message).
- Outside temp. sensor: An outside temperature value of – 40 °C is used to calculate the set flow temperature.
Part involved: Compressor fault (general message); Vitocal 300-G/333-G: Compressor fault (general message); Outside temp. sensor.
When it shows: Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Subject to other messages An outside temperature value of – 40 °C is used to calculate the set flow temperature.
Likely causes:
- Compressor fault (general message).
- Vitocal 300-G/333-G: Compressor fault (general message).
- Lead break, outside temperature sensor.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
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 heating system pressure gauge reading and report a low reading to your contractor; make sure the filling loop is closed.
- Check the outdoor unit (air source models) is not blocked by leaves, debris or snow.
What the engineer will check
- See further messages regarding the compressor and inverter (message code "80" and above).
- Check resistance value (NTC 10 k Ω) at plug F0: See "Controller and sensor PCB".
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 167, 173, 161, 86-87 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026