Possible Causes

Defective defrost heater, Failed defrost thermostat, Faulty defrost sensor, Main control board not initiating defrost

How to Fix / Troubleshooting

Safety First: Unplug the refrigerator and prepare for water from melting ice.

Steps:

Manual defrost: Remove freezer contents and rear panel. Use a hair dryer on low to melt ice from the evaporator and surrounding components.
Test defrost heater: With the unit unplugged, disconnect heater leads and check continuity. Replace if open.
Check defrost thermostat: Test continuity when the thermostat is cold (frosted). Replace if open when cold.
Inspect defrost sensor: Measure thermistor resistance and compare to expected values. Replace if out of range.
Control board: If all components test good, the main PCB may be failing to start defrost cycles and may need replacement.