Depending on which model, most have two evaporators, two blower fans inside and two defrost systems. Clearly the control board controls all that. If it's nor cooling in only the reefer section, it might be the control board. Or a single thermister (thermostat), fan motor, damper motor, defrost component (heater, limit, or thermistor), freon control valve could be the culprit. Many of the new units have diagnostic algorithms that even the home owner can run. Just depends on which unit. Some (Samsung, LG) even have an app that the owner can hold their phone near an upper hinge to run diagnostics. Some of the Whirlpool models have a single UI at the top of the reefer (front center) and only a single evaporator in the freezer section with a fan blowing air into the reefer section above. Toss us out the model#.