Before buying parts for a lot of money, just do a media calibration. Sometimes, for some reasons, because of media, hardware- or environmental issues, the OPOS-Sensors sensitivity needs to be recalibrated.
Print a rectangle of 5 to 5cm, treat it like your original prints (lamination etc.), place it in the plotter, select CAL MEDIA on the plotters menu (the position of this menu item depends on the exact plotter model), it will ask you to place the sensor on the black area or white area ... after that the plotter will save a recalibration value OR will give an error message if the contrast can't be calibrated.
But, depending on the OPOS-Generation (OPOS or OPOS X), the sensor will run into problems with modern digital printers (first generation one time developed for screen printing in 1994).