38 on a Viessmann Vitotronic 200: Flow sensor primary: Operation with temperature value of return temperature sensor in primary circuit, plus 3 K.; If the primary circuit flow and return temperature sensors are both faulty at …
Short answer
On a Viessmann heat pump, 38 means: Flow sensor primary: Operation with temperature value of return temperature sensor in primary circuit, plus 3 K.; If the primary circuit flow and return temperature sensors are both faulty at …. 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 manual says: Flow sensor primary: Operation with temperature value of return temperature sensor in primary circuit, plus 3 K.; If the primary circuit flow and return temperature sensors are both faulty at ….
Part involved: Flow sensor primary.
When it shows: Operation with temperature value of return temperature sensor in primary circuit, plus 3 K.; If the primary circuit flow and return temperature sensors are both faulty at the same time, the message "A9 Heat pump" is displayed and the heat ….
Likely causes:
- Lead break, flow temperature sensor, primary circuit (heat pump air or brine inlet).
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 per code; Vitotronic messages remain until the cause is removed and are acknowledged with OK (6150133 p.82). Unremedied acknowledged faults are redisplayed the following day.
Manual reset: Acknowledge with OK on the programming unit (user level permitted per 5817913 p.56-57) after the cause is remedied.
What you can check
- 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 resistance value (Pt500A) at terminals X25.1/ X25.2: See "Controller and sensor PCB".
Who to ask for: Viessmann-trained heat pump installer/service engineer.
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 90 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026