A passive car alarm turns on automatically when the key is removed from the ignition and the car doors are closed. This type of alarm is considered "passive" because the driver does nothing to turn it on.
The features of a passive alarm may include flashing lights, audible alarms, a fuel system cutoff, an ignition cutoff, tracking devices, motion sensors and proximity sensors. These features ensure total safety of a car in order to prevent car theft. An active car alarm is another type of car alarm. Active alarms must be manually turned on by the driver in order for them to work.