Freezer evaporator covered in ice

Noticed early this week that our freezer wasn't cooling properly and some things in the fridge just above the freezer were frozen. Took off the panel in the back part of the freezer and found the evap coil was completely covered with ice. I did not take any readings with a multimeter but did do some tests in the diagnostic service mode. The evap thermistor was reading -11 degrees Fahrenheit. I replaced the defrost heating element after thawing the whole freezer out. Turned back on and is cooling for about 15 hours. I don't see much frost at all on the coil (no ice at all). I turned on the FZ defrost via diagnostic mode but it didn't seem like the glass tube heated up at all. Would the thermistor still report an accurate temperature if it was broken? Would turning on the defrost heating element via diagnostic mode bypass the thermistor entirely? Should I be testing the power to the heating element when it is supposed to be under load?