This post is for a Jenn-Air bottom freezer, french door refrigerator model JFC2087HRS.
Last week, my refrigerator compartment was warm, but my freezer was cold. Frost was building up on the freezer compartment back panel. I removed the panel: lots of ice and frost on the evaporator coils. I defrosted it with a hair dryer and removed the evaporator heater and defrost thermostat. The heater checked ok for continuity. After placing the thermostat into my other freezer for about 30 minutes, I checked for continuity, and there was continuity. (I did not check the freezer thermistor.) After reassembling everything, cold air was getting up to the fresh food compartment. Now, the problem is reoccurring; frost buildup on the back of the freezer compartment and the fresh food compartment is warm.

Could the thermistor be the culprit? How do I check at the control board, so that I don't have to cut the wires to the thermistor? (I have the wiring diagram, but I don't know which pins on the connector to probe for a resistance check. My tech sheet gives me the resistance specs at 77 degrees F (10K ohms), 36 degrees F (29.5K ohms), and 0 degrees F (86.3K ohms).)

If the thermistor checks ok, how do I test the control board? Obviously, I don't want to order the control board if the thermistor is bad. Thanks!

More info: I ran the Forced Defrost Mode per the tech sheet. The display showed Fd, and I confirmed it by pressing the - key once. The refrigerator LCD showed 5, but the tech sheet says it should show S. I then pressed the - key to confirm. No defrost is occurring, and the evaporator heater is not warm.