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Sub-Zero Defrost Cycle / Freezer Not Freezing Issue


Jun 14, 2019
Los Angeles
Model Number
More than 10 years
I am troubleshooting my Sub-Zero 661 and before calling in authorized Tech (that is at least $1000), I'm trying to isolate the problem here. Unit had varying temperatures in Freezer. Some parts normal, some areas not getting cold (door compartments). Found evaporator fan/blade hindered by ice buildup on evaporator coils. I manually defrosted them. Is the photo below normal after resetting unit? Is that how evaporator coils normally frost over? Or do I have a defrost cycle problem? There isn't a defrost timer on this 661. The thermistors send signals to the control board, which turns on/off the defrost cycle.

This coil is only getting the freezer down to 19 degrees, so something is wonky.

If true, the culprits may be:

1. Themistors
2. Defrost heat element
3. Control board
4. Something with the closed system.

I have to decide whether to repair this unit, or call it a day with Sub-Zero, unit is 12 years old and I'm told they're supposd to last 20 (at $10K per, they ought to!).

Any insight helpful:

I'm never crazy about being the bearer of bad news but that's a sealed system problem. It could be a refrigerant leak or system restriction. Either way it's an expensive repair and will often exceed the machines value. In LA (and other major cities) the repair costs can be more than the cost of a new machine.

I'm going to have a tech visit to confirm this, but before I do... I can throw the unit into manual defrost mode, which shuts down the compressor and shuts off the evaporator. At the same time, it's supposed to fire up a heating element that defrosts the evaporator coils. When doing so, everything that's supposed to happen, happens EXCEPT for the heating element. It doesn't change temperature at all. I suppose the next test is to see if there's voltage across the element, but have you heard of the (or seen) heating element going bad? There seems to be a lot of places that stock the part, like it's something that does happen on Sub-Zero units.

Last year, we had the freezer's compressor replaced and the entire system flushed and recharged... I can't find any leaks or indication of leaks, but that's a naked-eye examination.
With adaptive defrost, even in manual defrost if the defrost sensor doesn't think it's cold enough to start a defrost it won't send voltage to the heating element There's also a defrost terminator that closes at a set temperature in line with one of the wires going to the defrost heater. If it doesn't close the circuit for the element is broken. Check the element for continuity and check the defrost terminator for continuity if both test good the board is not sending the voltage to the element. However, that is not a defrost problem. It is a sealed system issue. The evaporator is starving.
MFMauceri said:
I can't find any leaks or indication of leaks
A refrigerant leak is only one possibility. It can also be caused by a partial restriction in the cap tube or filter drier. It can also be caused by an inefficient compressor. It's not common and since you had the compressor replaced it's more likely to be a refrigerant leak or system restriction.
MFMauceri said:
......and shuts off the evaporator
The evaporator has no electrical parts. It's just a set of coils the refrigerant flows through.
MFMauceri said:
I'm told they're supposd to last 20
Whoever told you that, lied. It's not uncommon for machines manufactured before 1990 to last that long because there's no "electronics" installed in them. Today's machines it's all about saving energy. The ¼ hp R12 compressor has been replaced by a 1/10 or 1/12 R134a compressor. R12 was very forgiving. R134a is not.

This is the frost pattern of a machine with a defrost problem:

This is the frost pattern of the evaporator with a sealed system problem:
Thank you for this very informative post and responses. I have only one last saving grace.... When I first defrosted the evaporator (blow dryer), it was completely covered with frost like the "Defrost" examples in photos. I'm not getting any error messages from the control panel... None. All thermistors reporting back temperature readings ( I read the service manual) and the freezer evaporator reports back 0 degrees, while the comaprtment reports back 19 degrees. I've heard the freezer compressor kick off on its own volition and the fan cut off at the same time-- like it's trying to go into defrost mode. So, that leaves closed system problem and thermostat/defrost problem. I will keep running it to see if the frost spreads... If it's closed system problem... I'm going to assume it's a restriction, and I'm not sure how that gets fixed without big $.

Which mfgs do you like that aren't Sub-Zero? $10K is a lot to throw down for a refrigerator.

Or is it a case where they all suck and some suck less?

BTW Sub-Zero told me that their units should get about 20 years of service when I was looking at which models replaced this unit in case I had to buy and swap.

If it comes to that, like hell will they "cart" it away.

I'm stripping the mother down for every working part someone might want, and they can cart away the icebox (sans doors).

I missed the part where you already defrosted the coils. So how long has it been running since you defrosted it and took the picture? Do you have the evaporator cover installed? If the cover is off it won't cool properly and the frost builds quickly and migrates down the evaporator. If the cover is installed and the evaporator fan is working and the machine was running for at least 24 hours before you took that picture, without a doubt you have a sealed system problem.
MFMauceri said:
... I've heard the freezer compressor kick off on its own volition and the fan cut off at the same time-- like it's trying to go into defrost mode
OK, here's the deal; When the compressor turns on the condensor fan motor turns on and the evaporator fan motor turns on. When the the machine reaches the set temperature and cycles off, both fans turn off. Whenever the compressor is running the fans will be running. The only time the evaporator fan motor runs without the compressor running is when the control senses the freezer door is open. When the door is closed the evaporator fan turns on for 5 minutes to cut down the condensation on the glass door and shelves. When the machine starts a defrost cycle the compressor and both fans turn off. At the end of the defrost cycle there's a 5 minute delay before the compressor and fans turn on while the drain tube heater remains energized There's no way to tell just by listening if the compressor and fans cycled off because it reached set temperature or if it's starting a defrost cycle.
Again, thanks Rick!

The freezer was not cooling optimally. It wasn't a BIG problem because the temperature was in the teens. When the fan blade was clanking, I thought it was the motor and/or fan blade... no. It was just hitting the ice block on the evaporator. After melting the ice and restarting, I was waiting for the temperature to get down to zero--now, two days--before replacing the cover-- I also wanted to leave it off for repair tech's convenience.

However, I will manually (blow dryer) defrost and replace cover and see what happens?..

The evaporator is reaching temperature (if that temperature is zero), because the diagnostic says it is. Hoping it is NOT a closed system issue. My next thought is that the whole evaporator isn't getting cold because it's freezing up. When I defrosted it with the hair dryer, the WHOLE evaporator was a block of ice.

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