In both cases it was the defrost timer, an electric clock switch that turns the defrost on & off. On these, you can check this by manually advancing the timer until it goes into defrost. On my ~8 year old Amana, the timer can be advanced at a recessed screwdriver-slotted red knob on the ceiling of the frig portion just behind the panel with the temp controls. If yours has this, advance the knob & see if the defrost turns on (you can tell because the switch clicks and if the compressor is running at the time, it shuts off).
Easy and cheap to replace, if that's the problem. Also if the thing's iced up you can defrost it a couple times this way and get air moving again until you can replace the part.
Other guess could not thermostat. That doesn't control defrost. First check for
power on the defrost heater. If none then check deforst timer for power
in. If present then change defrost timer. That is where power comes from
to the heater. If power then bad heater.
Thermostat controls the compressor. When the temp inside goes up the
thermostat turns on the compressor to cool the fridge or freezer. Here's the diagram. It looks like if the thermostat is "open", the heater won't run...
Also, in thius links, can check info...
-
applianceaid.com/defrost_cycle.html-
applianceaid.com/frig_notcold.html#ffHope helps.
