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Sub-Zero 550 Refrigerator is not cold enough


Premium Member
Jun 6, 2022
Model Number
More than 10 years
Unit: Subzeero 550, top refrigerator, bottom freezer, 29 years old
Problem: Refrigerator is not cold enough. Milk went bad before expiration date last week. Condensor fins were vacuumed several months ago.

I measured temperatures. All measurements were made while the compressor (smaller) was running. The other compressor was not running. Freezer has no problem.

Top shelf: 57 F (14 C), thermistor contact thermometer
Bottom shelf drawer: 43 F (6 C), same as above
Compressor: 129 F (54 C), non-contact IR thermometer
Condenser exhaust air: 96 F (35 C), same as above, condensor fan was running.
Line from compressor to condenser: 122 F (50 C), thermistor contact thermometer
Line coming out of refrigerator to compressor: 86 F (30 C), thermistor contact thermometer, thermistor held by hand and not accurate, feels not cold

The evaporator fan operates well when door switch was pushed in. What is going on with this refrigerator? Thank you.

EDIT: The bottom 1/3 of the back wall (behind the drawers) is frosted. Why is this happening?


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Follow-up diagnosis (I cannot edit the original post any longer)
Subzero lists air leaking into the refrigerator and clogged drain. Door seal seems to be tight. Apparently, the previous owner replaced it. I squirted water into the drain and noticed dripping in the pan at the bottom of the unit. At this point, I am suspicious of low freon charge that caused ice forming on the evaporator. Am I moving in the right direction? If so, what are my options? The tag says R12 refrigerant. Thank you.
Looking at your pic some one did system work on the ref side. When & what was done? Is the frost across the entire back panel or only on the lower left side.
Thank you for the reply. Previous repair makes sense. There is a sticker on the tag (see photo). I don't know what was done. If I find anything in the documents the previous owner passed on, I will update. Frosting/icing is for the entire width of the back wall behind the drawers, NOT left half.


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I found a YouTube for evaporator replacement on this model. Back wall frosting/icing matches the size/location of the evaporator.
That tag is the amount of replacement freon for the new style evaporator which must have been replaced. Your issue is with the control. They usually fail within a year after an evaporator replacement, I always change that. If you remove the back panel you will find the evap. encased in frost
I have a capillary thermostat. It has COLD OUT 2, COLD IN 6, WARM IN 13 (celsius). It does not list WARM OUT. Since the difference between COLD IN and COLD OUT is 4 degrees, may I assume that WARM OUT would be about 4 degrees above WARM IN?

Thank you.
I am looking at a generic capillary thermostat. Exact fit costs $180 at ebay but a generic one costs $10. Thank you.
It is up and running. Thank you. It took a while to find a SZ parts dealer that covers my area.

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