Ok Finally Stuck, GE Fridge Fan motor only operates if thermistor is unpluged

[Toy.Tigers]

Problem: fan is not turning on in the refrigeration part, freezer is working great, reset the door controls and that did nothing, unpluged the themistor and the fan stays on and freezes everything, Unit seems to be in constant defrost but not sure. I believe the themositor should be a closed switch, and when cold opens, it this correct?

Replaced the main board, the fan, the THERMISTOR FF EVAP sensor. The thermostat, the only part not changed was the thermostat sensor, all parts have to come from the USA and hand delivered, so I am trying to get this working properly, The refrigerator is irreplaceable, We had to have a crane bring it up 8 stories and a dozen men reach out and catch it.... not for the faint of heart.... so I cant just go out and get another one.... got to trouble shoot this one. How do I test the thermostat sensor as this is the only piece that isn't new, or is there a reason the fan is off< the door switches are bypassed during the test.

GE PSB48YSXASS 

Parts replaced, 602,608, 609, And the main motherboard

https://www.geappliancep...0/0/0/FRESH_FOOD_SECTION

[16345Ed]

If it’s that important to you I advise hiring a qualified professional technician. 

[David Jero]

3 hours ago, 16345Ed said:

If it’s that important to you I advise hiring a qualified professional technician. 

I thought the same thing.  There is no point as a home owner with that much invested to try and save a few bucks when you have so much invested. 
But here is a quick tip for you. This GE Built in SXS refrigerator was actually built by Samsung. Knowing that. I would highly advise you call GE factory service directly.

As a prior GE tech who worked on many of these. You most likely have an ice ball up in the fresh food blower fan.  I would handle this repair by ordering the entire evaporator assembly. I wouldn't replace the evaporator. But all the parts that it comes with. It's a big job. And it would be best to be done by a professional.  Remember, this is only a guess. But it was very common.  Don't bother taking a short cut on this one. Will just create bigger problems that will lead into bigger issues.