HP CP5225 printing too dark - HEWLETT-PACKARD Color LaserJet CP5225 Printer

If a service person can do it you should be able to I think. See link below for assistance/support for your device
HP Color LaserJet Professional CP5225 Printer series

the transfer unit shold to replace

Close the right door

FIRMWARE - corrupted, you have to down load firmware.

PRESS HOME BUTTON & OPEN FOLLOWING MENUS.
ADMINISTRATION ~ MANAGE SUPPLIES ~ RESET SUPPLIES ~ NEW FUSER KIT,
THEN USE DOWEN ARROW BUTTON TO HIGHLIGHT " YES " ITEM THEN PRESS OK.

clean mirrors on the laser unit all four mirrord

HP has a list of solutions for print quality problems. Have you tried all the solutions suggested here?

Troubleshoot problem for HP Color LaserJet Professional CP5225 Printer...

There were also a couple Mac specific solutions available (links look the same but are different). Both solutions are about prints being different than expected.

Troubleshoot problem for HP Color LaserJet Professional CP5225 Printer...

Troubleshoot problem for HP Color LaserJet Professional CP5225 Printer...

I am an HP employee.

replace the cartridge or give it a good clean

Is the toner bonded on the page? If not then the fuser is likely faulty. Otherwise you might have a bad cartridge. If you run the print quality test pages you should be able to tell if it is just one cartridge that is bad.