Transfer units are used to transfer the toner from the image drum / imaging unit onto the paper using a charge that attracts the toner. When a transfer unit begins to fail, all of the toner on the image drum is not being transfered to the paper resulting in faded or incomplete images on the printouts. Okidata has designed you printer to have its transfer unit replaced when it has exceeded its life span.
On Oki printers you get a warning that your printer’s transfer belt or transfer roller is coming to its end of life and will need to be replaced. Once your transfer roller or belt as exceeded its life your printer may stop printing to protect the printer from any damage that may result from using an expended transfer unit.