Sorry for the delay.
Remove the rear panel in the freezer and check the defrost heater and the defrost thermostat for continuity. The thermostat has to be cold at the test time.
If the defrost heater is bad, then the new one comes together with the thermostat, so you do not have to order it.
If both, the heater and the thermostat checked OK, then the problem is a bad defrost timer (#257 on the
diagram).
- The defrost thermostat
Part number: AP2071262 
- The defrost heater assembly
Part number: AP2071523 
- The defrost timer
Part number: AP2061695 
Gene.