Have you checked the fuser?
A laser printer fuser assembly is the heating roller unit that melts and fuses the toner onto the paper. The fuser assembly is made up of a heating roller and a pressure roller.
The heating roller applies the heat to the toner on the printed page and the pressure roller presses and fuses the toner into the printed page. If it's bad and not heating properly, the print will be faint, which is what you describe. You may be able to remove and clean the fuser, but if the printer gets lots of use, it may just be worn out.
See this link for an idea of what it looks like and a price.
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