The Evaporator Coils are Frosted Over
If the freezer is cold but the refrigerator is warm and the evaporator coils are frosted over, there may be a defrost system problem. Several times a day the defrost heater assembly turns on for a few minutes to melt away any frost or ice that may have accumulated on the evaporator coils. If the defrost heater assembly has burned out, or the defrost thermostat or defrost control have failed the frost will build up on the coils and eventually no air will be able to travel through the coils to provide cooling. The most common cause for this condition is a problem with the defrost system.
Evaporator Fan Motor
If the refrigerator freezer is cold but the refrigerator is warm, the evaporator fan motor might have failed. Every refrigerator has a set of coils called an evaporator. These are the coils that get cold. A small fan and motor is mounted near the coils. This fan and motor - called the evaporator fan and motor, draws air over the evaporator coils and circulates it throughout the freezer and refrigerator. If the fan fails, the freezer or refrigerator will get much warmer than usual.
Damper Control Assembly
If the freezer is cold but refrigerator is warm the air damper control might be stuck closed or broken. The air damper control is either an automatic or manually operated door which opens and closes to let more or less cold air into the refrigerator compartment. If it doesn't open properly it won't let enough cold air into the refrigerator. If the evaporator fan is running, the damper door is open and no air is coming out. The evaporator may be frosted up caused by a defrost problem.
If the freezer is cold but the refrigerator is warm the thermistor might be defective. The thermistor is a sensor which monitors the air temperature. It is connected to the control board. If the thermistor is defective the refrigerator does not cool or may cool continuously.
Temperature Control Board
If the refrigerator's freezer is cold but the refrigerator is warm the temperature control board might be defective. The temperature control board provides the voltage to the fan motors and compressor. These boards are often misdiagnosed. Check all other components to be certain this is the cause of the problem.
If the refrigerator freezer is cold but the refrigerator is warm, the defrost thermostat might be defective. Several times a day the defrost heater turns on for a few minutes to melt away any frost that may have accumulated on the evaporator coils. In order for this to work, the defrost thermostat has to sense that the coils are cold enough. If the coils are cold enough the defrost heater will turn on. If the thermostat is defective it may not be able to sense the temperature of the coils and then won't turn on the defrost heater. If the defrost heater doesn't turn on, the frost will build up on the coils and eventually no air will be able to travel through the coils to provide cooling to the freezer or refrigerator. The defrost thermostat can be checked for continuity. It should have continuity when it reaches the low temperature of its operating range.
Main Control Board
If the refrigerator's freezer is cold but the refrigerator is warm the main control board might be defective. This is not common. The cause of this problem is almost always in the defrost components.