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Heat pumps · Error codes · Viessmann
40Viessmann · Vitotronic 200

40 on a Viessmann Vitotronic 200: Flow sensor HC2: Heating circuit mixer M2/HC2 is closed

Short answer

On a Viessmann heat pump, 40 means: Flow sensor HC2: Heating circuit mixer M2/HC2 is closed. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.

Reviewed 23 September 2026 · 1 source

What it means

The manual says: Flow sensor HC2: Heating circuit mixer M2/HC2 is closed.

Part involved: Flow sensor HC2.

When it shows: Heating circuit mixer M2/HC2 is closed.

Likely causes:

  • Short circuit, flow temperature sensor, heating circuit with mixer M2/HC2.

Is it safe?

Does the heat pump stop?Not stated
Engineer needed?Yes

The manual does not say whether the heat pump stops.

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 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 (NTC 10 k Ω) at plug F12: See "Controller and sensor PCB".

Who to ask for: Viessmann-trained heat pump installer/service engineer.

What you might notice

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026