A0 on a Viessmann Vitotronic 200: Current sensor phase L1 of compressor supply voltage faulty
Short answer
On a Viessmann heat pump, A0 means: Current sensor phase L1 of compressor supply voltage faulty. 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:
- Current sensor phase L1 of compressor supply voltage faulty.
- Vitocal 300-G/333-G: Current sensor phase L1 of compressor supply voltage faulty.
- Ventilation. Check filter(s): Mechanical ventilation in control mode.
Part involved: Current sensor phase L1 of compressor supply voltage faulty; Vitocal 300-G/333-G: Current sensor phase L1 of compressor supply voltage faulty; Ventilation. Check filter(s).
When it shows: Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor OFF Mechanical ventilation in control mode.
Likely causes:
- Current sensor phase L1 of compressor supply voltage faulty.
- Vitocal 300-G/333-G: Current sensor phase L1 of compressor supply voltage faulty.
- Outdoor air and extract air filter on ventilation unit contaminated.
- Last filter change was more than 1 year ago.
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 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
- Replace inverter.
- Vitovent 200-C/300-F: Replace outdoor air and extract air filters. Do not clean filters.
- Vitovent 200-W/300-C/300-W: Clean outdoor air and extract air filters. In the event of severe contamination replace both filters at least once every 12 months.
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 171, 178, 98 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026