The circulation or evaporator fan usually is located in the freezer compartment. It keeps air circulating from the freezer to the refrigerator. The purpose of this fan is twofold: to prevent frost from building up on the evaporator unit, which is also located in the freezer, and to keep the air in the fridge cool. The cold air travels through a vent found near the top of the refrigerator.
To test for operation, open the refrigerator door and hold your hand in front of the vent. You will feel cold air blowing out of the vent if the fan is operating properly. If you don't feel the air moving, and the refrigerator compartment feels warm, or you see frost or ice building up on the evaporator unit in the freezer, the fan's motor likely is broken and will need to be replaced by a professional.
Cooling or condenser fans are located either on the lower back of the fridge or underneath the refrigerator, above the water pan. This fan blows warm air over the condenser coils, which prevents frost from building on the surface of the coils. The warm air also evaporates the water that collects in the draining pan, which is a result of the refrigerator's self-defrosting unit.
Because of its location, this fan often gets covered with dirt and grime, which can hamper its movements and even halt it completely. To remedy this, unplug your refrigerator and vacuum the dust and debris from the fan with a hose attachment. Vacuum debris from the evaporator coils as well. If the fan still does not move after you plug the refrigerator back in, the motor probably is broken and will need to be repaired by a professional.