X740x/750x/760x series uses four colors and has a imaging unit for each one: yellow(Y), magenta(M), cyan(C), and black(K).
K740x/750x/760x/765x series uses one color and has a imaging unit for black(K).
When the life of the imaging unit expires totally
The status LED turns red on and the display screen shows the error message for replacing imaging unit.
Check the type of the imaging unit for your machine.(see Consumables and Accessories)
At this stage, the imaging unit needs to be replaced.