66 on a Viessmann Vitotronic 200: Motor overload relay or safety equipment at the soft starter has responded
Short answer
On a Viessmann heat pump, 66 means: Motor overload relay or safety equipment at the soft starter 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:
- Motor overload relay or safety equipment at the soft starter has responded.
- Swimming pool flow t.: Operation with temperature value of flow temperature sensor in the secondary circuit of the lead heat pump.
Part involved: Motor overload relay or safety equipment at the soft starter has responded.; Swimming pool flow t.
When it shows: Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor off. The other compressor may remain in operation. Operation with temperature value of flow temperature sensor in the secondary circuit of the lead heat pump.
Likely causes:
- Motor overload relay or safety equipment at the soft starter has responded.
- Short circuit/lead break, swimming pool flow 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 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
- Check soft starter. Check status input at the controller PCB: See "Controller PCB and EEV PCB [6] ".
- Check motor currents/motor overload relay.
- Check resistance value (NTC 20 k Ω) at plug F21: 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 182, 94 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026