When calibrating, the Pan axis moves towards the one end of travel. Upon reaching the end of travel the fixture continues to drive the motor and then stops after a few seconds.
The end of travel Sensor in the pan unit is a proximity switch which senses when a metal flag is pushed into it's path. If the fixture does not see this flag it will drive the motor until it senses the axis has stopped without sensing the end of travel sensor, and then shuts off the axis.
To remedy this, verify the wire harness to the end of travel sensor is intact. If not, you will need to replace the end of travel sensor.
Make sure plastic gear is not damaged. There is a metal plate that should spin freely. If this plate doesn't, it will cause the fixture to calibrate incorrectly.