See the attachment for your tech sheet.

Force a defrost, see the tech sheet.
Be sure that the defrost thermostat is frozen.

If the defrost heater comes on then replace the control board.

If the heater does not come on.
Check the defrost heater, should be around 25 ohms.
Check the defrost thermostat should be 0 ohms when frozen and about 56,000 ohms when unfrozen.
Use your correct meter scale so you get actual resistance readings.
Short the meter leads together before starting so that you can see if there is a zero offset in the meter.
Also it is a good idea to unplug at least one side of any part you are measuring for resistance. This ensures that you do not read an alternate/parallel circuit.
If both OK then as a double check you could check the heater/thermostat combo at P1- 3 to P1-2 at the control board.This will check the parts and the wiring.
If both parts are OK then replace the control board.

Note that if the defrost thermostat is deformed/bulged replace it even if it measures OK.