I have a Samsung RS2555SW Side by Side Fridge. Recently the cooling side quit cooling and i had the defrost heating unit replaced which had iced up and quit working. That solved a problem for a while. Then the cooling side started freezing everything even though the temp indicated 4 degrees. The thermostat or temp sensor was checked and appeared to be working ok. It stopped freezing and worked ok for a week or so and then started freezing everything again. It stopped freezing things but now appears to have quit cooling (at least it is not as cold as what the temp indicator says). I have not had problems with the freezer side but now the freezer temp is flashing at -21 and it does not feel as cold as it says. The unit is making an abnormal loud buzzing noise and the fan is making quite a noise. It fan noise stops when you open the door. This fridge is only about 8 yrs old.

Hello,
Your fridge not working is a fridge repair job that you can do without the help of a professional. Often the fridge has a frost-free failure, or the compressor is faulty but there are many other reasons for a fridge not cooling properly.
Step 1 - Find the Problem

Your first job is to locate the source of the problem. Consider each of the following:

* Gaskets - Make sure the gaskets/seals are not torn and are sealing properly.
* Evaporator fan - Make sure it is working. If it is now working you may have a problem with the fan motor. Check for power to the motor and also examine the fan switches around the door (not all fridges have these switches).
* Air damper - Sometimes the air damper will not open up to allow the cold air from the freezer part to blow into the fridge section.
* Condenser coils - Check to see that the condenser coils don't have a dust build-up and that the condenser motor is working.
* Compressor - Make sure your compressor is working with on and off clicking noises.

Step 2 - Frost-Free Failure

This is the most common cooling problem in fridges. Expose the evaporator coils by accessing the panel in the freezer section. The coils in the freezer section can sometimes get clogged up with frost. This can stop the evaporator fan motor from blowing cold air around, or cause the fan to hit it and stop, or become noisy. The defrost timer can be a bit tricky to find but it is usually located behind the back bottom corners of the fridge, although they can sometimes be found in the ceiling of the fresh food section, or behind the cold control cover.
Step 3 - Find the Timer

When you find the timer, turn the wheel-like screw slowly with a screwdriver until the fridge shuts off. The refrigerator is now defrosting. If you find that the fridge starts now, you will need to replace the defrost thermostat and the defrost timer.
Step 4 - Volt Test

If the heaters do not come on, use a volt meter to ohm test the defrost heater or volt test for 120 volts to the heater. You can bypass the defrost thermostat if you haven't got power to the defrost heater to check if the defrost heater will come on. To bypass the thermostat, join the two wires together. Replace the defrost thermostat and the defrost timer if the heater comes on now.

If there is a ticking or squealing noise in the defrost timer, or it seems hot to touch, replace it.

Many fridges have and inline fuse on both sides of the defrost heater. You will need to replace the whole heater if one of those fuses blows. Check these fuses with a volt meter if the defrost heater doesn't work.

A quick check if you have a cooling problem is to inspect the evaporator coils. If there is a build up of white snow on the coils, this indicates a frost free problem; if there is balled ice on part of the coils with the rest bare, this indicates a system problem, like a problem with the pumping compressor.

Hope this help in solving the problem. Take care.....

I ran across this web blog about this model refrigerator, and perhaps it might help:
http://www.justanswer.com/questions/2rk7s-i-have-a-samsung-rs2555sw-fridge-sxs-which-seems-to-be-cycling
Not sure if you need this, but here is a site with diagrams and parts lists for your model:
http://www.appliancepartspros.com/partsearch/model.aspx?model_id=5174704
Here is a web page that tells how to troubleshoot a thermostat
http://www.ehow.com/how_2167621_troubleshoot-refrigerators-thermostat.html
You might check your control board for any cracks or bad solder joints. This problem sounds like an intermittent connection or failiing part, but could be either.
Here's a general troublshooting site, and it covers different situations that may assist you:
http://www.repairclinic.com/Refrigerator-Troubleshooting
Good luck, and hope this helps.