GE PRS 23 Evaporator Frozen FF-Side Warm

The fresh food side has had an issue cooling for years (maybe since it was new). It was set to 34 and generally ran high 30s to low 50s, and there was mold everywhere and small cracks in the interior case (nice for $3k). The freezer side (separate evaporator) seems fine as it is at a consistent 0 degrees.

I disassembled the FF side for defrosting, cleaning, and drying. The evaporator was a solid block of ice. The evaporator fan runs fine.

Using generic diagnostic codes found in the web:
A. I ran sensor test 07 and all thermistors passed, and also measured OK with an ohmmeter.
B. I ran defrost test 14, and there was no apparent defrosting, in fact the evaporator was cooling very well while in this test.

There is no heater on the FF evaporator, just a thermistor in an aluminum block tie wrapped to the coils. With what means does it defrost?

Having eliminated the thermistors, and in the apparent absence of a evaporator defrost heater on the FF side, does this problem sound like a main board issue? WR49X10152 - correct?

Can anyone send a troubleshooting manual? Any other input?

SN: RG 43 06 89

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Thanks!

Just in case anyone else needs a PSH23 Service Guide, I found this:
[COLOR=#0066cc]http://www.mediafire.com/file/z0zzbihwzew/31-9118.pdf[/COLOR]