I have been having defrost issues for several months on my Frigidaire. It started after a burning smell that seemed to be coming from the fridge. I have replaced the Temp Control Board because that went out. I changed the Defrost Therm. The defrost heater is reading 30ohms so I decided to change the main board thinking that would solve the problem. Well, it didn’t. Does anyone have any other suggestions?
Here are your parts
http://www.appliancepartspros.com/parts-for-frigidaire-ffus2613ls0.html
Here is the tech sheet.
http://manuals.frigidaire.com/prodinfo_pdf/Anderson/240389639.pdf
What type of defrost problems are you having.
FYI: The wiring diagram shows a relay in the compressor and defrost circuit.
I cannot find this relay in the parts.
Also it does not make sense to me as it looks like the compressor and defrost heater are both turned on when the relay contacts close.
It is no defrosting. I tried to turn on the manual defrost but that also does not work. The fresh food side of the fridge was warm and that is how I noticed the evaporator coils were frosted over. I manually have been defrost coils with a heat gun but that is getting old. The only part left to change is the defrost heater itself but based on the continuity test heater should be good.
I would check the continuity of the defrost system.
Make sure that the defrost thermostat, evaporator coil is frozen.
Unplug the unit.
Measure from J1-11 at the defrost board (Neutral) to J1-7 (heater/thermostat combo).
It should read about 30 ohms.
This then checks all components including the wiring in the defrost circuit.
Measured from J1-11 at the defrost board (Neutral) to J1-7 (heater/thermostat combo) tonight and it read 30ohms. 
At the top of my list would be a bad defrost board.
At the top of my list would be a bad defrost board.
I had the defrost issue before I replaced the temp control board. The temp control board is new. I then changed the main control board and it is still not defrosting. What is the part # for the defrost board?
Sorry but I did not look up the info on this unit before answering.
Since you said defrost I just used that term but there is not a separate defrost board in this unit.
The correct board is
AP5645865