I have a Trane XL 1200 Model TWX030C100A0 ser. # HO4202122 that has been a superb unit but now fails to initiate the defrost cycle. I can cause the unit to defrost by putting it into the AC mode when it frosts up. The defrost board is 21C140501G07 (WW28X0329). I have not replaced this board in the unit but this number board has been replaced in the Trane inventory by a board numbered CIB2515A. I haven't traced the location of the defrost thermostat yet but understand it is located on the lower part of the unit and connects to the circuit board by two black wires with female spade connectors to the circuit board black plastic module with male spade connectors. Is there a way to determine if the defrost thermostat is the problem or the circuit board or both (or something else)?
Thank you,
C. L. Rogers

Replace the defrost thermostat first because it is cheapest. If that fails replace the board. When a board fails I always replace the defrost thermostat anyway.

The answer to your question is yes they can be teated BUT as a homeowner there is no way to be accurate. Toss the defrost thermostat into the freezer then check with ohm meter. The failure here is that the defrost thermostat is designed for a specific temperature and you cannot be that accurate by placing it in the freezer at home.

My Trane TWX030C100A0 blew a hole in the compressor. Should I put in a new compressor (and other parts) or buy a new unit?