Faint prints are caused by insufficient toner transfer. It's not the cartridge.
FIX: Enable Toner Saving mode. This is a driver setting. On Windows, open Printing Preferences, go to the 'Advanced' tab, and check 'Toner Save'. On a Mac, look for a 'Quality & Media' or 'Advanced' setting in the print dialog.
IF THAT DIDN'T WORK: The drum unit (DR-2400) may be near end of life. Its rated yield is approx. 12,000 pages. Replace the drum unit if page count is high.
HEADS UP: Toner Saving mode reduces toner usage to lower costs. For normal prints, keep it off. Also, ensure you removed all sealing tape from the new toner cartridge.