Thermostat Defects
One reason your freezer fan may not turn off is because the thermostat is defective. The thermostat tells the system when to cool based on the temperature setting you select on the dial inside the refrigerator. If the thermostat is not accurately reading the internal temperature of the refrigerator, it may signal the system to run a cooling cycle permanently. This will make the fans run constant and waste and a considerable amount of energy.
Door Left Open
If the refrigerator cannot reach its desired temperature, it will continue to run a cooling cycle and will therefore keep the freezer fan circulating the chilled air it is creating. When you leave the door ajar on the refrigerator or freezer, it allows cold air to escape and quickly raises the internal temperature of the refrigerator. Close your doors properly so there is no way the chilled air can escape. Once it cools down to the desired temperature, the system will shut off and the fan will stop.
Gaskets
Around the edge of each door on your refrigerator is a rubber seal known as a gasket. The gasket is what makes the door to the freezer and fresh food compartment airtight. If this gasket gets dirty or is torn or otherwise damaged in any way, it may not allow the door to close completely and may allow cold air to seep out into the room. When this happens, the refrigerator will have a hard time keeping up with cooling demand and may constantly circulate fans.
Defrost System Failure
Another cooling problem that may result in a constantly running fan is a defrost system failure. Self defrosting refrigerators clear frost from the evaporator coil in the freezer by using a timer and heater. The timer tells the heater to come on for a set period of time to melt away the frost. When the defrost cycle isn’t on, the cooling cycle is running. If the timer gets stuck in the cooling cycle the heater will not come on and frost will build up on the coil. Eventually, this build up will restrict airflow to the rest of the refrigerator and will cause the cooling cycle and fans to run continually in an attempt to cool down the interior.