Freezer evaporator coil freezing up

powered off, removed contents and shelving from freezer, using nutdriver removed 4 screws holding cover in rear of freezer, lifted cover from bottom and slid towards front, unplugged 2 wire leads from fan motor, unplugged wiring harness top left at plug and removed cover, unplugged 2 leads on clip-on defrost thermostat and removed from copper tubing, replaced with new part in reverse order, unplugged wire lead on left and right side of evaporator coil leading to defrost heater, slid out left side of evaporator coil carefully and unfastened clip from defrost heater, rotated up right side defrost heater and removed element, attached clips to new defrost heater and installed in reverse order, attached left and right wire leads, plugged in fan motor leads and wiring harness, replaced cover tilting top to front and sliding in bottom first. installed 4 screws, replaced shelving and powered on. within 3 hours freezer was 0 degrees, fridge 39 degrees, replaced shelving and contents.