A refrigerator defrost system is the set of parts in a frost-free refrigerator that periodically melts frost off the freezer's evaporator coil. A timer or control board switches on a defrost heater, a defrost thermostat shuts it off once the coil warms, and the meltwater runs through a drain to a pan under the unit.
Why it matters to you
When the defrost system fails, frost builds up on the evaporator coil and blocks airflow, so the fresh-food section warms first while the freezer may still feel cold. Spoiled food is often the first sign, before any noise or error code.
Practical guidance
Look for heavy frost on the freezer's back panel, a warm fresh-food section, or water pooling under the crisper drawers from a frozen defrost drain. Unplugging the unit to thaw the coil is only a stopgap; a failed heater, thermostat, or control needs diagnosis.