E1 on a Viessmann Vitotronic 200: External heat source: Heat pump control unit cannot start external heat source
Short answer
On a Viessmann heat pump, E1 means: External heat source: Heat pump control unit cannot start external heat source. 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: External heat source: Heat pump control unit cannot start external heat source.
Part involved: External heat source.
When it shows: Heat pump control unit cannot start external heat source.
Likely causes:
- External heat generator fault.
- Short circuit/lead break, boiler water temperature sensor in external heat generator.
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 external heat source.
- Check resistance value (NTC 10 k Ω) at plug F20: See "Controller and sensor PCB".
Who to ask for: Viessmann-trained heat pump installer/service engineer.
Sources
- Service instructions Vitotronic 200 type WO1C heat pump control unit (air/water split & monobloc, brine/water) (6150133, 2023-07) ↗ · 6150133 VSA00004 · page 108-109 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026