There are a couple possibilities here. This units defrost limiter is actually a fuse. wr50x10047
It is in the defrost circuit. make sure your evapator fan is running. This can cause a light frost pattern on back panel as well as a poor cool issue. Also, did board installation call for the clipping of thermistor wire? Make sure you have ir flow to fresh food area. Do so with freezer door shut.Make sure fresh food light is turning off when door is closed.....With unit in an obvious iced up defrost condition remove and Jumper the (L1 to defrost) terminals on the three wire quick disconnect on main board. You should see an arc. If so, I would think main board is bad. If no arc then your element or limeter fuse is bad. most of these units use the wr50x10068
defrost limit switch is used. It is resettable L140-30. Your unit uses instead wr50x10047
THERMODISC(FUSEM) this is non resettable. I do not know its limit. Good luck.