Dehumidifier displays E4 and will not run; sometimes appears after long operation or defrost cycle.

Safety: Disconnect from mains before opening the cabinet.

• Locate defrost sensor: On Medisana dehumidifiers, the defrost sensor is often clipped to the lower part of the evaporator coil or attached to the refrigerant tubing.
• Check attachment: Make sure the sensor is in firm contact with the coil/tube. If it has fallen off, reattach using the original clip or aluminum tape rated for refrigeration use.
• Inspect cable and plug: Follow the cable back to the PCB. Ensure the connector is fully seated and there are no visible breaks.
• Replace sensor if needed: If the sensor is physically damaged or tests open/short with a multimeter, replace it with the correct Medisana defrost NTC sensor.
• Board diagnosis: If a known-good sensor does not clear E4, the main control PCB is likely defective and should be replaced by a qualified technician.