98 on a Viessmann Vitotronic 200: Voltage difference of individual phases too high (> 5 A)
Short answer
On a Viessmann heat pump, 98 means: Voltage difference of individual phases too high (> 5 A). 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:
- Voltage difference of individual phases too high (> 5 A).
- Solar module sensor 7: No control of the appliance (circulation pump or 3-way diverter valve) connected to plug sS on the solar control module, type SM1.
Part involved: Voltage difference of individual phases too high (> 5 A); Solar module sensor 7.
When it shows: Type "Note" (H) - triggers "07 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor OFF No control of the appliance (circulation pump or 3-way diverter valve) connected to plug sS on the solar control module, type SM1.
Likely causes:
- Voltage difference of individual phases too high (> 5 A).
- Short circuit, temperature sensor / at solar control module, type SM1.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Warning |
| 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.
What the engineer will check
- Replace inverter.
- Check sensor / on solar control module, type SM1: See installation and service instructions for solar control module, type SM1.
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 160, 165, 97 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026