This may be referring to the end of paper sensor.
It seems to be an intermittent issue that may be caused by the paper ,paper
core spinning on the mandrel. The mandrel may not be far enough into the the
paper core. Another check, take any credit card. Try , without force to put it
in between the metal mandrel tube and the right side of the plotter. If it
doesn't go in between, this is the correct condition. If it fits in. There is
too much space, the end of paper encoder won't read the pulses correctly,
presenting the issue you are experiencing.