Turbo cool will not turn off in GE side-by-side refrigerator. Replaced circuit board...still stays on

There are three temp sensors in my GE side-by-side model GSS20IEMDWW (two in the freezer and one in the fresh food side). Replacing the temp sensor in the fresh food side was enough to allow the compressor to finally turn off.

For me, the problem manifested with an iced-over evaporator coil inside the freezer compartment. Since the compressor never cycled off (presumably because the temp sensor was providing faulty data about the temp inside the fresh food side, thus causing it to continually attempt to cool), the coils were constantly chilled. Eventually the ice on the coils would block the air flow and the freezer side could no longer freeze

I purchased a pair of inexpensive thermometers from a kitchen store and put one in the freeze and one in the fresh food side so I could occasionally look at the temps to see if it made sense for the compressor to be running - ie, was it really a warm-enough temp for cooling to be required, or was the computer reading faulty sensor data.

Six months later I also needed to replace both temp sensors in the freezer compartment. My refrigerator is about seven years old. I would recommend replacing all of the temp sensors just to rule that out.

http://www.partselect.com/AdvancedModelSearch.aspx?ModelNum=GSS20IEMDWW&mfgModelNum=&fkMfgID=2

Your model may be different, but this will give you an idea of part numbers and relative locations of the parts.