E4
Keystone Defrost / Frost Protection Sensor Error or Overload Protection

Keystone portable or window air conditioner shows E4 and may stop cooling or enter protection mode.

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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

Defrost sensor failure, sensor out of position on evaporator, iced-up evaporator causing abnormal readings, control board logic fault

How to Fix / Troubleshooting

Safety: Turn the unit off and unplug it. Allow any ice to melt before working near the coil.

Steps:

  • Let the unit thaw: If you see ice on the evaporator coil, leave the unit unplugged with the front cover open for several hours until all ice melts. Ensure the room is not extremely cold and that vents are not blocked.
  • Clean filter and coil: Wash the air filter and gently vacuum dust from the evaporator coil fins to restore airflow.
  • Locate defrost/frost sensor: It is usually clipped to the evaporator coil tubing. Confirm it is firmly attached and not dangling in free air.
  • Inspect wiring: Check the sensor harness for damage and reseat the connector on the control board.
  • Test sensor: With power disconnected, measure resistance and compare to other thermistors at similar temperature. Replace if open/shorted or clearly out of range.
  • Restart: After reassembly, plug in and run the unit on COOL with a moderate set temperature. If E4 returns quickly with a good sensor, the main PCB may need replacement.
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Repair Difficulty

Medium 3/5

Required Part

Defrost / Frost Sensor (Evaporator Thermistor)
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