Hi, first time posting to the forums and have a question/problem. For the past couple of months our 30o Sub Zero 550 refrigerator has been unable to keep a steady temperature. Using two wireless bluetooth thermometers (cheap Amazon kind, they're great) and a plain mercury thermometer, I can see the temp swing from 50 degrees during the day to 25 degrees at night. This is with the fridge compartment temp setting dial turned to "3".

It seems like the thermostat is off. I can turn the desired temp setting to "10" briefly, which causes the cooling cycle to kick in within a second, then I turn it back to "3". That will cool the fridge down successfully so the cooling system itself works.

I then wake up at 3am to my bluetooth temp alarm saying "Fridge is below desired temp". Gotta go upstairs, turn the knob to "1", which causes the cooling cycle to stop within a second, then I turn it back to "3". That will allow the fridge to warm up successfully so our watermelons don't freeze and turn to mush.

I'm getting tired of babysitting this thing. It's $10000 or something to replace it so I think it's worth my time/effort to diagnose. If we replace it we'd go with something cheaper.

There's an additional issue with the fridge drain line freezing up. Probably due to the freezer insulation below losing its ability to insulate the drain line from the freezer compartment. I have tried various tricks (including a heating wire, and hot water spa treatments) to keep this line clear. That seems solvable but even with the line clear, the temperature issue persists.

Do I just need a new thermostat?

Thanks for any help/advice! Love these forums, thanks for all the help already on here.