Stadler Form dehumidifier shows E4 and will not start the compressor or fan

Safety: Unplug the unit. Do not attempt to open the sealed refrigeration system yourself; refrigerant work must be done by a licensed technician.

• Allow full defrost: If the unit has been running in a cold room, move it to a warmer area (above 18°C / 64°F), unplug it, and let it sit for 4–6 hours to fully defrost. Then restart and see if E4 clears.
• Check airflow: Clean the air intake and outlet grilles with a vacuum and soft brush. Remove and wash the air filter with mild soap and water, dry completely, and reinstall. Blocked airflow can cause coil temperature errors.
• Inspect evaporator and condenser: With covers removed, check for heavy dust on the condenser (warm coil) and ice buildup on the evaporator. Clean dust with a soft brush and vacuum. Do not bend the fins.
• Sensor verification: Ensure the evaporator NTC sensor is correctly attached to the coil and functioning (see E2 procedure). A bad sensor can trigger E4.
• Pressure switch (if present): Some Stadler Form dehumidifiers use a pressure or thermal protector on the compressor line. Check its wiring and connectors for damage or loose contacts.
• When to call a technician: If E4 persists after cleaning, defrosting, and sensor checks, the unit may have a sealed system issue (low refrigerant, blocked capillary, or compressor problem). This requires professional diagnosis and is often not economical to repair on small domestic units.