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Ask Randy 43 Your Own Question
Randy 43
Randy 43, Home Appliance Technician
Category: Appliance
Satisfied Customers: 3764
Experience:  20 yrs experience 18 in home,gas license , refrigeration expert
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I have a Sub Zero Model 550 with the freezer in the bottom.

Customer Question

I have a Sub Zero Model 550 with the freezer in the bottom. The fridge works great all the time. The freezer forms frost and ice on the top, which causes it to not maintain it's temperature even if you keep lowering the control. I have defrosted it manually many times, but each time it takes about 4 to 6 weeks and the situation repeats itself.
JA: Can you guesstimate how old your Sub-Zero is? And just to clarify, what's the exact model?
Customer: It is a Model 550 that was built in 1993.
JA: What are all the symptoms of the problem (error codes, flashing lights, etc.)?
Customer: I don't know of any error codes or flashing lights. The earliest symptom is that ice cream no longer stays frozen really hard, instead becoming somewhat soft. After a week or so like this, the ice maker will stop working because the freezer isn't cold enough.
JA: What happened just before this problem started? What troubleshooting have you tried?
Customer: Nothing. It has been doing this for a long time. I have cleaned the condenser on top. I fully defrost it about 6 or 8 times a year in order to make it work. I have inspected the gasket, which seems fine, and have put lubrication on it to keep it soft as instructed by a technician a few years ago.
JA: Anything else you want the appliance expert to know before I connect you?
Customer: No.
Submitted: 9 months ago.
Category: Appliance
Expert:  Randy 43 replied 9 months ago.

hello , if it is a full frost on the ceiling that is a defrost issue for the freezer ,most likely the defrost thermostat or heater or the timer ..the timer is up top behind the grill ,it puts the freezer into defrost 3 times over 24 hours ,being that old ,it most likely has pitted contacts and should be replaced regardless ..the other two parts are in the freezer behind that ceiling cover and can be checked for continuity with a meter ..thermosta is a little bulb that closes when it gets cold and the heater is is strapped onto the coil will be one of those 3 parts that are defective causing the ice build up ..i would change the timer regardless ..and check the other two and replace if no continuity

The defrost thermostat monitors the temperature of the evaporator coils. When the coils drop below a set temperature, the thermostat contacts close to allow power to flow to the defrost heater during the defrost cycle. In the defrost cycle, the defrost heater melts away any frost that may have accumulated on the evaporator coils. If the defrost thermostat is defective, the thermostat contacts will not close. As a result, the thermostat won’t provide power to the defrost heater. To determine if the defrost thermostat is defective, use a multimeter to test it for continuity. If the defrost thermostat does not have continuity when it reaches the low temperature of its operating range, replace it.


Defrost Thermostat for your Sub Zero Refrigerator

RepairClinic Item #1514997