What if my Trane 3 ton 13 seer heat pump is unable to generate enough heat to reach the set point on the thermostat?
In the heat mode the larger copper pipe going to  the AC unit outdoors should get very hot, if not then the refrigerant might be low or the reversing valve which switches modes from Cool to Heat may be bypassing. the fact it shuts down tells me its getting overheated likely due to low refrigerant level.