On specific BISSELL Vacuums, there is a Dirt Sensor. The clean carpet sensor detects dirt being picked up while the vacuum is in use. When the vacuum is picking up dirt, the red light will come on. Occasionally, the lights (green and red) may alternate quickly back and forth if the area is only lightly soiled or the dirt is spread out. For the best cleaning, continue vacuuming until the green light remains constantly illuminated.