This troubleshooting guide is for the AltMill using the SLB-EXT, with inductive sensors. Most likely this will only apply for AltMill MK1 because the harnesses are not consolidated like the MK2.
Issue(s):
- Alarm 15 pops up while homing
Resolution:
- Make sure the Y-axis motor and sensor cables are connected properly as pairs. The Y1 sensor cable must be attached to the white connector above the black Y1 motor connector. The same applies for Y2.
- Jog the machine to the rear bump stops of the machine, so that the right and left Y-axes are at the same Y position. Then check that the sensors on the left and right Y-axis are sticking out the same amount from their respective end plates. Now you can try to home again. 
 Machine may've been out of square, alarm 15 is built into the SLB-EXT firmware’s dual homing feature to prevent the left and right Y-axis motors from skewing the machine and causing serious damage.