Fan cycling low and high
The freezer fan slows down due to the refrigerator section being at the desired setting but the freezer section still needing coolant . If the refrig section needed more coolant , the fan would speed up , forcing more air into the refrig section .
After replacing the heater , it could be in the defrost cycle . The thermistors tell the main control board , " hey , we're freezing , turn on some heat " , in which the heater comes on and defrosts the coils , until behind the rear freezer panel , can get up to 140 degrees . As it gets this hot , the frozen food in the freezer , is what keeps the freezer cold . After defrost , the fan comes on and blows the residual heat into the refrig sectio , until the coils get cold again . So , if the refrig section warms up temporarily , it's normal .