E4 on a Carrier AquaSnap / AquaSnap Plus 30AWH: NUI EEPROM failure: all installer/user settings revert to factory defaults after power cycle; no field fix - thermostat must be replaced
Short answer
On a Carrier heat pump, E4 means: NUI EEPROM failure: all installer/user settings revert to factory defaults after power cycle; no field fix - thermostat must be replaced. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: NUI EEPROM failure: all installer/user settings revert to factory defaults after power cycle; no field fix - thermostat must be replaced.
Part involved: NUI 33AW-CS1.
Likely causes:
- NUI non-volatile memory failed.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
What you can check
- Note the code/LED pattern and the model from the rating plate, then contact a heat pump/refrigeration engineer.
- Check the heat pump's breaker/isolator at the consumer unit is on.
What the engineer will check
- Replace NUI user interface; re-enter configuration.
Who to ask for: refrigeration / heat pump engineer.
Certification: Electrical competence for work inside the unit.
What you might notice
Sources
- Carrier User Interface Comfort Series (NUI 33AW-CS1) Installation Manual, February 2010 ↗ · February 2010 · page 16 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026