Went to a call in Perry, Ga about a refrigerator ( Frigidaire GLRT185TDW0 ) that was not cooling properly. I found that the evaporator coils had frozen up. I manually put the unit in defrost mode and nothing happened. I grabbed the hair dryer out of the truck and started defrosting the coils. Once I had enough defrosted to test the defrost thermostat I found that the defrost thermostat was bad. The defrost thermostat allows voltage through to the defrost heater. Being the thermostat was stuck in the open position the heater couldn’t come on and melt the frost build up when the unit went in defrost mode. I replaced the defrost thermostat and tested for proper operation.