When your Imaging unit (also called a print unit) prints for a set amount of pages it will prompt you with the message "UNIT LIFE X" warning on the LCD display and stop printing. If your Imaging unit is still perfectly good, why waste the money for a new one? Just replace the reset chip.
Replacing the chip will reset the imaging counter back to zero and you can continue to print.
One chip required per imaging unit. Each chip lasts for one cycle, after which they must be replaced again.