I can only add that the heat exchanger (condenser) needs to be free of dust and its fan observed to ensure that it actually running at full speed. Then pressure issues can become the center of attention.

Hi, Yes Condenser is free of dust and fan is running.

How low pressure is causing the compressor to keep running?

Thanks.

Presumably, low pressure would lead to low refrigeration performance so the control system would try to keep the compressor running to make up for the low performance.

I understand low refrigerant pressure in the compressor.

But Temperature inside the fridge is below 38F (34F - 37F). thermostat should be turn off the compressor. NO?

i dont see any connection between low refrigerant in the compressor and thermostat.

I assume thermostat checks if temperature goes below 38F then turn off the compressor.

is my assumption right?

When we sold our former home, our SZ 36” over under fridge was still going strong at 25 years old. We were fortunate to live 20 minutes from a SZ distributor. When there was a problem, we’d call SZ, describe the issue, and we’d go pick up the part from the distributor. My husband became quite proficient at diagnosing and repairing that fridge.

I read your post to him, and he suggested that after 30 years, you might need a new compressor.

I will tell you, the new SZ fridges are more efficient, energy-wise. The electricity cost per month with our 4 year old 42” FD fridge, is far less than our old fridge, even though it’s larger.

After 30 years, it might be time to bite the bullet, and buy a new SZ.

"is my assumption right?"

This is also my understanding of a basic refrigeration system. Is your SZ a single compressor system? I thought that they all had dual compressors, one for freezer and one for refrigerator. (Mine does, anyway.) With a single compressor, simple refrigerator/freezers can control the two temperatures in different possible ways. Some are more sophisticated than others. I won't claim to know all of them. And in any case, without a schematic and voltmeter, I would have trouble even trying to diagnose what is going on. Maybe your control board has an issue.

Mine SZ has dual compressor. freezer and fridge separate compressors.

freezer compressor turns off.

let me ask same question

i dont see any connection between low refrigerant in the compressor and thermostat.

I assume thermostat checks if temperature goes below 38F then turn off the compressor.

is my assumption right?

All Sub Zeros have dual compressors. They were the innovators of dual compressor refrigeration.

@Qasim Khan, my husband fully admits he’s not a pro. He was giving his best guess, after hearing your issue. Why not ask SZ to send one of their technicians to examine and diagnose the problem? They will know best what to do.

I tried to contacted my local SZ dealer and they said, they only service those units which they sold. they will not come out to check my unit.

as per temperature inside the fridge below 38F, compressor and Freon looks good.

the only thing is compressor is not tuning off which temperature is below 38F.

Like i said if turn on number down 6 to 5 compressor turns off,

Where are you located? You should be able to find appliance repair companies in your area which will repair a sub zero refrigerator.

Mr. Appliance, for example, at least in my area claims SZ capability. (I haven't used them for this, however.) I believe Mr. Appliance is a franchise, and there should be others scattered about the country.

My fan on sub zero is not turning should I buy a new motor for fan

First, make sure that the motor is the problem by pushing on the fan blade with a stick and see if the bearings are frozen or if the motor can then run, but just not start. In either case a new motor (may come with fan) will be needed. If the motor turns freely but doesn't start, then one should check for voltage at the motor when it should be running. No voltage, possible board failure or relay failure. If you need the refrigerator to cool, then it is best to blow clean or vacuum the heat exchanger (condenser) and put a small fan into the space on one side or the other if it will fit to keep air flowing over the coils. I can only extrapolate from my 632, and have no idea how the other models are constructed.