Dehumidifier displays E4 and shuts down; compressor will not start.

Safety: Unplug the unit before inspection. Do not attempt to open the sealed refrigeration system yourself.

Basic checks for E4:

• Clean the air filter and grille: Remove and wash the filter with mild soap and water. Ensure the front and rear grilles are free of dust and obstructions.
• Check room conditions: Keystone dehumidifiers are typically rated for operation above about 41°F (5°C). If the room is too cold, the coil may freeze and trigger E4. Move the unit to a warmer area.
• Inspect the evaporator coil: Remove the front cover and look at the coil. If it is heavily iced over, allow it to thaw completely (leave unplugged for several hours) and then restart.
• Verify fan operation: Plug in and run the unit briefly (if possible) to confirm the fan runs at full speed. A weak or non-running fan can cause coil freeze.
• Sensor check: If E4 appears immediately on startup, the evaporator temperature sensor may be faulty or disconnected. Inspect and reseat the sensor as in the E2 procedure.
• When to call a technician: If airflow is good, room temperature is normal, and the sensor is intact but E4 persists, the issue may be low refrigerant or a failing compressor. These require EPA-certified service and specialized tools.