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Sub Zero freezer ice build up in drain pan


Premium Member
Mar 5, 2020
Model Number
Sub Zero
More than 10 years
Before I start into this, please note that I am not trying to DIY this repair, but before bringing out the repair company yet again, I want to have a better understanding of what might be going on. We've been working with the same tech for all of the repairs. The primary issue is with the freezer but the functioning of the refrigerator isn't perfect either. Please excuse me if I don't properly describe all of the repairs.

We have a 30 year old pair of 501 Sub Zero Refrigerator/Freezers. Both generally functioned when we bought the house a couple years ago, but had internal climate control issues. The refrigerator had issues with internal moisture and minor mold growth and the freezer had some ice build up.

We initially had the repair tech look at the refrigerator. They added an additional thermostat as it was assumed that internal temperature differentials were causing the moisture.
A few months later we realized that the constant flooding of the bottom shelf of the refrigerator was due to a frozen drain pan. They replaced the defrost heater about a year ago and haven't had that issue since.
However we started having issues with ice build up on the back of the refrigerator, and behind the back panel. They came out, reviewed several times, removed the ice, made a few minor component adjustments, but never really solved the problem. By trial and error we determined that whenever the kitchen temp raised above 70 degrees, we had to lower the internal temp from 5 to 3. While set on 3, we never have ice issues, but the internal temp measures at around 42, which is higher than we would like.

The freezer had internal ice and freezing issues also. The tech replaced both defrost heaters. The week after the heaters were replaced the compressor failed and had to be replaced. The new compressor had some sort of fault and also failed but we have not had any issues with that since.
However, the ice issue doesn't seem to have gone away. We realized late last year that the lower drawers were starting to get stuck. This was because the drain pan had completely iced over and had iced the bottom shelf as well. The tech came out dethawed the interior, and made a few adjustments. He thought that it was odd that it had taken months for this to happen and that it may have just been an unusual circumstance. But now less than 2 months later the drain pan is frozen again and is starting to overflow.
So main question is: what else can be done? What else should they be looking into?

Also before anyone asks the rational of repairing versus replacing units of this age, it was the tech's recommendation to repair versus replace as any of the components could be easily swapped out. Also due to the dimensions of these legacy units, any new refrigerator (and due to cost we would not do another 6' wide built in) would require an extensive renovation of at least one side of our kitchen.

Thanks in advance!


Appliance Tech - Moderator
Staff member
Jul 11, 2006
Murray, Utah 84107, United States
The freezer evaporator uses a defrost heater and a drain tube heater. The defrost drain water is caught by the V-shaped drain trough. Because of the the colder temperatures, a drain trough heater keeps the area warm during the defrost cycle only. If the heater for the drain tube is good and the drain trough is icing over the drain tube needs to cleaned out.
sub zero 501F freezer.jpg

The refrigerator evaporator doesn't use a defrost heater. This evaporator coil incorporates an off-cycle defrost. Once the system is pulled down and the control is satisfied, the unit is shut down. The unit will remain off until the 39° F cut-in temperature is reached at the control.
sub zero 501F refr.jpg

A frost issue on this evaporator is caused by either a sealed system issue or cold control issue. A refrigerant leak can cause an abnormal frost build up and since you had the compressor replaced that's the first place I would look.
Water in the refrigerator section is usually caused by a blocked drain line.
The tech came out dethawed the interior, and made a few adjustments.
made a few minor component adjustments,
Other than turning the cold control knob colder or warmer, there are no adjustable components in this machine.