BB on a Viessmann Vitotronic 200: Fault, PWM signal from secondary pump (actual speed)
Short answer
On a Viessmann heat pump, BB means: Fault, PWM signal from secondary pump (actual speed). The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manuals for this range word it in different ways:
- Fault, PWM signal from secondary pump (actual speed).
- KM BUS mixer cooling: No cooling mode.
Part involved: Fault, PWM signal from secondary pump (actual speed); KM BUS mixer cooling.
When it shows: Type "Fault" (S) - triggers "05 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor remains in operation. Secondary pump continues to run at previously set speed, without monitoring. No cooling mode.
Likely causes:
- Fault, PWM signal from secondary pump (actual speed).
- Communication fault KM BUS or internal fault in extension kit, NC-Box for cooling circuit. (cont.).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running.
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 room thermostat/remote control and time-program settings are as expected.
What the engineer will check
- Check PWM signal at secondary pump. Check signal at connection J26 on the EEV PCB: See "EEV PCB [4-6] / [4-7] ". Replace the secondary ….
- Check connections and parameter settings.
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 · page 179 · Reviewed 23 September 2026
- Service instructions Vitotronic 200 heat pump control unit (5692896, 2011-02) – 222-G/242-G/242-S/333-G/343-G/350-A ↗ · 5692896 VSA00001 · page 61-62 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026