E4
Nibe
Defrost cycle malfunction
E4 alarm, defrost function fault on air‑source Nibe furnace/heat pump
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Faulty defrost sensor, blocked outdoor coil, incorrect refrigerant charge, control board fault
How to Fix / Troubleshooting
Safety: Turn off power before cleaning the outdoor unit. Beware of sharp fins.
- Inspect outdoor coil: Check for heavy ice buildup, leaves, or dirt blocking airflow. Gently remove debris and melt ice with warm (not boiling) water if necessary.
- Check defrost sensor: Locate the coil temperature sensor on the outdoor unit and ensure it is firmly attached and wired correctly.
- Verify fan operation: Ensure the outdoor fan runs during heating. A non‑running fan can cause excessive icing.
- Reset alarm: After cleaning and checks, reset the E4 alarm and monitor the next defrost cycle.
If defrost remains ineffective, a technician must check refrigerant charge and control logic.
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Repair Difficulty
Hard
4/5
Required Part
Defrost Temperature Sensor
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