By thermister do you mean the defrost sensor? It is usually next to the evaporator. The sensor can be checked for continuity using an Ohmmeter. If it tests o/c (shows no continuity) at the operating temperature (around 0 degrees F), replace it.
If you do mean this part and since you have replaced it and the control board as well it may well be a wiring fault between the two or perhaps a faulty plug terminal connection either on the sensor or the control board.
If you cannot see the correct wires on the control board connector that go to the sensor to prove continuity between the sensor and the control board (do this with the power removed from the fridge) perhaps you could try using a Voltmeter across the sensor terminals when the fridge is running and see if you get a reading. As an o/c (open circuit) stops the defrost function, if the sensor tests OK then as it is a variable resistor there must be current flowing through it coming from the control board, therefore there must be a voltage drop across it. If there is no voltage measured across the sensor there is a wiring or board problem and since you have replaced the board.... Try not to "short out" or "bridge" the two sensor terminals with the meter test probes as you do not wish to cause a problem back on the control board.
Here is a link that in general shows where it is. Not sure about your model as i cannot find any information with the model number you posted.