there should be no frost on the back wall of the freezer
frost on the back wall means the evaporator is not defrosting
defrost system test must be done while the fridge is plugged in and cold
so if the unit is not cold, plug it back in overnight
then... remove the back wall of the freezer, you should see a block of frost from top to bottom on the evaporator, if you do remove the plastic kick panel on the front, on the left side is a round hole, inside the hole is the manual advance for the defrost timer, turn this with a screwdriver, it will click and the fridge will stop running, wait a few minutes and see if the heater starts to melt the frost, if it does you need to change that timer, if the heater does not get warm the next step is to bypass the defrost therm
to do this you unplug the fridge, then... near the top of the evaporator you will see a disk with 2 wires, on your generation of friggy you can unplug the wires to the therm and join the connectors that went to the therm togeather, after you join these 2 wires you should have the thermostat with 2 wires/connectors that go nowhere...
now plug fridge in again and wait, if the heater comes on now you need to change that defrost thermostat
NOTE!!!! DO NOT!!! leave this thermostat bypassed for any extended period of time, it is a safety device and leaving it bypassed could cause the heater to melt the fridge or start a fire!!!!! NOTE!!!!!
if after this bypass you still get no heat from the defrost element you need to test the element with a meter and then the timer with a meter
also, frigidaire defrost therms will look like the plastic top is popping off of them sometimes when they are defective, so if the therm looks like this it should be changed