If you have a multimeter, you can test the defrost thermistor for proper resistance.

Set the multimeter to the ohm setting.

Trace the two wires of the defrost thermistor to its connector.

Disconnect the connector and measure the resistance between the two terminals on the connector.

The resistance of the thermistor should be about 11kΩ at room temperature.

Here is a chart showing the values. It is based on the model LFX25973, but the values are useful.