I don't know this particular one but on some older machines, they had a transfer wire or foam transfer roller immediately underneath the toner cartridge. This produces a charge which pulls the toner onto the paper before going through the fuser unit (a hot roller) to fix the toner to the paper, if the charge is weak then that would cause a faint image as less toner was being transferred.
The transfer wire could be cleaned using alcohol and cotton bud, the foam roller type would need to be replaced, if that didn't fix it then things get more complex.