I have found that removing the frost with a hair dryer works for about two days , then they The coils frost over again. I read they threads,and trouble shooting post and found the defroster thermostat reading bad , I replaced it and am still having the same problem, How can I check the adaptive defrost control board to see if it is the problem ?
Here are your parts
http://www.appliancepartspros.com/partsearch/modelsearch.aspx?model=Frs26lf7dsr
Here is the tech sheet it has how to force a defrost cycle.
http://manuals.frigidaire.com/prodinfo_pdf/Anderson/240389616.pdf
If the defrost heater comes on then the board is bad.