Product review (5 stars): Tick tock defrost
Standup GE freezer coils kept freezing over and then freezer would start to warm up. Changed thermostat and no change. Replaced the timer and that heater fired right up in defrost mode. Do yourself a favor and put a mark on the timer and see if it actually moves in a couple of hours.  Will save you a lot of trouble if you start here. Most of the time frozen coils caused by tstat, timer and lastly heater.