Side by Side freezer coil iceing over

Freezer coil iced over two months ago. Replaced defrost heater only. Wire to one side of heater showed overheating marks on the insulator but the solderless connector looked ok. Refrigerator worked until now, icing over again. Checked continuity of heater element and it’s OK. Fan is working so I defrosted and started unit. Cools for awhile and then starts to ice over again. Should I have replaced the thermostat defrost device and/or the sensor temp ff… Would the board be at fault. Temperature in freezer section stayed around 10 to 20 degrees and the refreigerator section around 35 to 40degrees with both stats set at #3. Could the board be at faullt. Thank you for your help!.