Aprilaire dehumidifier displays E4, and it will not start the compressor.

Safety first: Turn off power before inspecting internal components.

• Verify ambient temperature: Check the room or crawlspace temperature. Most Aprilaire dehumidifiers require ambient temperatures above about 50°F (10°C). If the space is too cold, relocate the unit or add supplemental heat.
• Check airflow: Ensure that filters and grilles are clean and unobstructed. Restricted airflow can cause the coil to freeze and trigger E4.
• Inspect coil: Remove the access panel and look for ice buildup on the evaporator coil. If present, leave the unit off until all ice melts and correct airflow issues.
• Check coil sensor: Confirm that the coil temperature sensor is firmly attached to the coil and that the wiring is intact. Replace the Aprilaire coil temperature sensor if it reads out of range.
• Professional diagnosis: If E4 recurs with proper temperature and airflow, a refrigerant problem may exist. Have an HVAC technician evaluate the sealed system.