E691 on a Viessmann Vitocal 100-A: Low pressure transducer failure (blocks: Machine)
Short answer
On a Viessmann heat pump, E691 means: Low pressure transducer failure (blocks: Machine). 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: Low pressure transducer failure (blocks: Machine).
Part involved: Low pressure transducer failure.
When it shows: The alarm is triggered if any connected or enabled probe is short-circuited or interrupted. The alarm also triggers if the upper limit (100°C) or the lower limit (-50°C) of the probes is exceeded. A probe figured as sanitary probe will not trigger any alarms if ….
Likely causes:
- The alarm is triggered if any connected or enabled probe is short-circuited or interrupted. The alarm also triggers if the upper limit (100°C) or the ….
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated.
Manual reset: Placing the controller in OFF mode resets alarms and hour counters (6167577 s.13, printed p.34); if alarms persist when switched on, contact technical assistance.
What you can check
- Note the E-code (e.g. E006) shown on the Vitocal 100-A controller display and give it to your installer.
- Per 6167577 s.13, switching the controller to OFF resets alarms; if the alarm is still present when switched back on, stop and contact your installer/Viessmann technical assistance (do not keep cycling it).
- Check the heat pump breaker/isolator in the consumer unit is ON and the outdoor unit is not blocked by leaves, debris or snow.
- Check the heating system pressure gauge and make sure the filling loop is closed.
What the engineer will check
- Check the related sensor/transducer wiring and value (probe short-circuit/open circuit or outside -50..100 °C).
- Read alarm log (Hist) on controller; contact Viessmann technical assistance if alarm persists.
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification.
Certification: F-gas certificate (UK F-gas Regulations; Viessmann: refrigerant work by authorised refrigeration engineers only).
Sources
- Installation/service instructions Vitocal 100-A outdoor unit controller (6167577, 9/2020) ↗ · 6167577 VSA00001 · page 36 (PDF p.38) table 13.2 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026