Trane split system heatpunp, outside unit model AWX030C10040, S/N G07297149, MANFACTURE DATE 2/92. Defrost board 21C140501G07, WW28X0329.

OUTSIDE FAN MOTOR DOES NOT RUN (WILL RUN IF YOU BYPASS THE DEFROST RELAY ON THE DEFROST BOARD.) Compressor runs but stays in the defrost mode all the time. How do you check the defrost thermostat. It checks open with a ohm meter tester. Should it read resistance depending on temperature or is it an open or closed contact depending on temperature? What is the purpose of the other sensor that plugs into the defrost board (outdoor temperature sensor) ?

Should I replace defrost board, and both sensors or is there a way to determine which is bad?

You have a bad board if its stuck in defrost mode. The defrost thermostat if open will not let the defrost board go into defrost and in turn its what takes it out of defrost when opening.