Sub Zero Refrigerator Runs Constantly – How to fix? by HoustonAuthorizedRepair.com 

The part(s) or condition(s) listed below for the symptom Refrigerator not cooling are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.

Most Frequent Causes for Refrigerator Runs Constantly

Cause 1 Condenser Coils are Dirty
The condenser coils are usually located under the refrigerator. They dissipate heat from the refrigerator as refrigerant passes through them. If debris accumulates on the coils, the refrigerator will have to work harder to remove the heat. If the coils are significantly dirty, the refrigerator will run constantly in an effort to cool down. Your refrigerator’s condenser coils should be cleaned twice a year.
Cause 2 Freezer Temperature is Above 10 Degrees F (-12C)
If the freezer temperature is above 10 degrees Fahrenheit (-12C), the refrigerator will not be cold enough. If the refrigerator is not cold enough, it will work harder to cold down. As a result, it will run constantly. The freezer temperature should be set between 0 and 5 degrees Fahrenheit (-18 to -15C). If the freezer temperature is too high, ensure that the condenser coils are clear of debris and the condenser fan is working properly. In addition, check to see if frost has accumulated on the evaporator coils. If the evaporator coils are frosted over, part of the defrost system has likely failed.
Cause 3 Defrost Heater Assembly
The defrost heater assembly turns on several times throughout the day to melt away any frost that may have accumulated on the evaporator coils. If the defrost heater assembly is defective, the refrigerator will not defrost, and frost will continue to accumulate on the evaporator coils. If the evaporator coils are frosted over, the refrigerator will work harder to remove the heat. As a result, the refrigerator will run constantly in an effort to cool down. To determine if the defrost heater is defective, use a multimeter to test it for continuity. If the defrost heater does not have continuity, replace it.
Cause 4 Defrost Thermostat
If the refrigerator runs constantly, 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 freezer or refrigerator 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 . The defrost thermostat can be checked for continuity. It should have continuity when it reaches the low temperature of its operating range.
Cause 5 Door Gasket
If the Refrigerator runs constantly the door gasket may be torn or defective, or if the door is not shutting tightly warm air can enter the the refrigerator and cause the evaporator coils to ice over. This happens because humid air from the room is drawn into the refrigerator through the opening at the gasket. As the humidity comes in contact with the cold evaporator coils it condenses and freezes. Normally the defrost system removes this frost and ice. However, if the door gasket is defective the evaporator coils ice over too quickly and the defrost cycle cannot keep up.
Cause 6 Evaporator Fan Motor
If the refrigerator runs constantly, the evaporator fan motor might have failed. Every refrigerator has a set of coils called an evaporator. The refrigerator may have one or more evaporator fan motor(s) depending on the model and evaporator locations. The evaporator fan motor circulates the cold air from the coils thru the compartment. If there is only one evaporator it is in the freezer side. If the fan is not working, no cold air will get to the refrigerator compartment. The freezer may still get cold.
Cause 7 Defrost Control Board
If the refrigerator runs constantly, the defrost control board might be defective. The defrost control board monitors several conditions and determines how often to run the defrost cycle. If the board fails, the refrigerator will not go into the defrost cycle. Eventually, ice will build up on the evaporator coil and then the refrigerator and or freezer will not stay cold enough. This will depend on where the evaporator is located. It’s prudent to check the defrost heater and defrost thermostat for continuity before replacing the defrost control board.
Cause 8 Defrost Timer
If the refrigerator runs constantly, the defrost timer might be defective. Two or three times a day the defrost timer is supposed to turn on the defrost heater to melt away any frost that may have accumulated on the freezer evaporator coils. If the timer is defective it won’t advance and won’t turn on the heater.
Cause 9 Main Control Board
If the refrigerator runs constantly, the main control board might be defective. A few times a day the control board is supposed to turn off the cooling system and turn on the defrost heater to melt away any frost that may have accumulated on the freezer evaporator coils. If the control board is defective it could cause the system to never cycle into defrost.
Cause 10 Condenser Fan Motor
If the refrigerator runs constantly there might be something caught in the condenser fan motor blade, or the condenser fan motor might be defective. The condenser fan motor draws air over the condenser coils to cool them. The condenser fan motor is located underneath the refrigerator near the back. Pull the refrigerator out from the wall and remove the access panel to access it.
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