A few things to check:
1) Make sure black strip is clean.
2) Sometimes overhead lighting can affect the sensor, so close the front cover if it isn't already.
3) Make sure you haven't pinched the wires that run from the sensor to the slider board. This sometimes happens when you are working in the area, for example when you are changing dampers...
4) Check if the sensor is aligned properly over the black strip. You can use the paper sensor test under the #TEST menu to check the actual readings of the sensor at various positions across the platen to see exactly where the problem is.
5) Make sure the media is set at least 15 mm from the right edge of the right-most pinch roller.
6) Bad sensor, slider board, or other cable/ circuit problem.
We have parts.
Rob Earle Inkjet Services
44 Venable Road
Harrisville, NH 03450
Authorized Mimaki Service and Parts