checking the board is too complicated without proper training, even in here.
Instead test continuity through your defrost heater element, you can remove it from the system to test it. the other part to test is the defrost be-metal thermostat, this part needs to be continuity tested while it is clipped to the freezing cold evaporator, otherwise you will get a false open circuit reading.
If both of these parts test ok, you p[robably have a bad mother board.