Hi,
Did you mean model # FFU145HWC?
Have you checked that the replacement defrost thermostat is still OK?
If you have a DMM (digital multimeter) use its’ Ohmmeter function to test the thermostat out of circuit.
Remove the thermostat and let it warm up to ambient temperature. The specifications say that it should have a resistance value of ~6.2K Ohms at 68 F. so you may have to allow a bit if the ambient temperature is higher or lower. The higher the temperature the lower the resistance value and conversely the lower the temperature the higher the resistance value
Next submerge it in a glass full of ice water, the temperature of which should be close to 32 F. The resistance of the thermostat should then measure ~16.3K Ohms.
If you aren’t getting these values (or close to them allowing for temperature variation) then it is faulty and needs to be replaced.
Check behind the kick plate to see if there is a “tech sheet” for the freezer.
I cannot find the specifications for the thermistor, which you may also have to test just to make sure that it is still OK. If there is a “tech sheet” it will have the specifications for the thermistor.
Sometimes replacement parts can go faulty so it is worthwhile to check and eliminate them and not assume that they are OK just because they’re new.