The Drum LED lights if the drum unit is near the end of its life. Use the drum unit until you have a print quality problem, then replace the drum unit with a new one.
The Drum LED continues to light if the drum counter was not reset when you replaced the drum. To reset the drum follow the instructions below:
Open the front cover.
Keep holding down the button until all LEDs turn on with the front cover open, then release the button.
Make sure that the Drum LED is off. Close the front cover.
Reset the drum counter only when you have replaced the drum unit with a new one.