How do I fix a Sub-Zero refrigerator that is not defrosting automatically?

To fix a Sub-Zero refrigerator that is not defrosting automatically, follow these steps:
1. Check the defrost timer: Test the defrost timer to ensure it is advancing correctly. If the timer is faulty, replace it.
2. Inspect the defrost heater: Verify if the defrost heater is working correctly. A defective heater should be replaced.
3. Check the defrost thermostat: Test the defrost thermostat for continuity. If it doesn't have continuity when cold, replace it.
4. Clean the condenser coils: Dust and debris on the coils can affect the defrosting process. Clean the coils using a coil brush or vacuum cleaner.
If the issue persists, consult a professional Sub-Zero technician for a thorough diagnosis and repair.