Note, this is a stand alone refrigerator with no freezer (Kenmore Pro model 253.44123800). We have a full size matching freezer next to the refrigerator.

The evaporator coils freeze up and it appears the unit never goes into defrost mode. The Sear's Repair guy has been out to the house multiple times and it still isn't working. I've watched the repair each time.

He initially checked the thermostat using cold spray to ensure it closed.
At the electronic control board wiring harness he shorted 2 pins and verified that the heating element came on which it did. (he also checked continuity via a meter beforehand).
He replaced the Refrigerator Electronic Control Board (p/n: 297334400, original p/n: 297112400)

Didn't work.

Next visit he went ahead and replaced the Defrost thermostat (p/n: 216731001) and another Electronic Control Board. Reason for swapping the control board again was the 1st time he didn't disconnect power to the unit. This time we popped the breaker which was easier than pulling out the built in refrigerator.

Still doesn't work.

This unit doesn't appear to have a defrost timer as it is controlled by the computer. There is also no way to force the unit into defrost that I know of (wish there was).
Only other thing I can think of is the wiring from the thermostat to the control board. Also, I wish I knew what the flow diagram was of the defrost software to ensure there isn't another part involved?