He's probably referring to the motherboard. It controls all the functions of the machine. Here's a parts break down to show you where it's found.Sealed system & mother board replacement parts for General Electric GTS22KBMBRCC Refrigerator | AppliancePartsPros.com
Part number: AP4436216

Be sure to unplug the refrigerator first.

It's static-sensitive, so you'll want to handle the board by the edges, and make sure you ground yourself by touching the metal of the refrigerator before you handle it.

As long as you replace the wiring, one connector at a time, removing it from the old board, and placing it onto the new one, you should takeno more than about 15 minutes to do the job.

Your original post said the refrigerator is "stuck in defrost". Remove the panel inside the freezer at the rear to expose the evaporator coil. The defrost heater is at the bottom of the evaporator (the evaporator is the aluminum coil in there). If the defrost heater is on, it will probably be glowing, and the frost build-up on the evaporator will be melting or all gone.

There is a thermister (that's a resistor whose resistance value changes with the temperature somewhere in the area of the evaporator (usually on the floor of the compartment) that should signal the motherboard that the correct temperature has been reached to terminate the defrost heat.

If the thermister is doing its job, the heater will be off.

Another thing you can do is visually check the motherboard. Look for any obvious signs of damage (broken solder joints, burnt or badly scorched components, etc).