Alan
Here is one more test you can do. disconnect the stepper at the control board. With the four wires not touching one another rotate the stepper. Also remove the stepper from the machine. Now turn the stepper. It should turn easily but you should feel it cog as it turns. It will not turn like say a bearing. Now connect the black and green wires together and turn the motor again. It should turn with a lot more resistance. Now undo those wires and connect the white and red together. You should feel the same resistance. If not one set of coils has a problem. You can also take VOM and check the resistance of the of the 2 coils. they should be very close to the same. You can also cross check between coils there should be no continuity between coils.Also check coils to motor case there should be no continuity there.
You can also do the same tests for one of the other stepper motors and compare result
Dave
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Thanks @DavidSohlstrom. Those are good tests for the motor. I have already checked for a short between the two coils and found no continuity between them. I can perform the other checks when I get back home tonight.
Was the Z motor running hot? I just find it surprising that a motor would fail. I’ve used, or more appropriately, abused stepper motors for years and never had one fair. I have purposely killed a nema 23 motor, testing how much voltage I could put through one. It failed (actually started smoking) around 65v. Did you try swapping maybe the Y motor for the Z motor and see if the problem follows the motor?
It did seem hotter than the other motors.
I did swap the Z and X motor connectors and the problem did follow the motor.
It seems odd to me also that the motor would fail, I checked all the wiring and the controller board first thinking it must be something other than the motor.
These are low cost stepper motors from China. I’m sure they do not test 100% so I think it very possible that this motor was bad from the factory.
They have to source the motors from China because in order to keep the cost down for the hobby user they can not source American made steppers at 4 to 6 times the price.
Dave
That makes sense, I just ordered a new NEMA 23 so hopefully it will fix the problem. If not then I am back to being very confused.
Always the voice of reason Dave. 
You are very likely correct. All the motors I have played with have been surplus, american and some Japanese, stepper motors.
