Here are the parts. You seem to be missing the last digit of the model number.
Replacement parts for FFU20FC6AW models | AppliancePartsPros.com

[COLOR="DarkRed"]How do I get the back off to check the theromstat and the heater?[/COLOR]
The following should help.
http://appliancedigest.c...995531505-service-manual
You have to join the site to download the manual but it is free and this is a very good site for manuals and other info for the appliance repair DIYer.



For the below I used AW4

First remove the evaporator cover in the freezer so you can see the coils.
Do not let them de-ice.
If they are heavily iced/frosted over you have a defrost problem.

You may not have to remove the cover just to determine if the defrost heater comes on, you may be able to see the glow or hear the water dripping/sizzling.

If yes.
Manually force a defrost cycle by turning the defrost timer cam (Item 23 in Section 3 ) till the fans and compressor turn off.
Now check the defrost heater to see if it is on.
Be careful you do not want to burn your fingers.
If the heater is on then the timer needs replacing, re: it is probably stalling during it's rotation so is never getting into a defrost cycle.

If not on.
Unplug the unit.
Remove the wire for one side of the heating element (Item 14 in Section 3) from the wiring and measure it for continuity, usually around 20 ohms or so.

If the heater is OK
Remove one wire to the defrost thermostat (Item 20 in Section 3) and measure it, should be 0 ohms when frozen. Note that it opens just above freezing so must be frozen to check it. Also inspect it, if it is bulged at all replace it even if it measures OK.

If both the above are OK then odds are the defrost timer contacts are toast.
Best way to test this is a live test to see if you have 120 volts across the heater/defrost thermostat combo.