AutoLoc's advanced EZ back up sensor sends ultrasonic waves into the space behind your vehicle when you are reversing. AutoLoc's revolutionary tri-sensor system will automatically detect and warn you of any objects that are hidden from your view by emitting a sequence of pulse tones. The closer and louder the tones get indicates the distance of the object from your car. Detailed instructions ensures easy installation even for the weekend mechanic. Backed by a Limited Lifetime Warranty.Please Note:Some vehicles may require a rubber grommet housing for sensors to function properly. Some surfaces (such as metal or chrome bumpers) may cause interference with the operation of this unit due to ultrasonic wave emissions, causing a false reading that may be corrected with the use of an appropriate grommet These grommets can, in some situations, protrude as much as a 1/4"" from the bumper surface. Appropriate grommets can often be obtained at your local hardware/automotive parts store.