High Quality Content by WIKIPEDIA articles! A car alarm is an electronic device installed in a vehicle in an attempt to discourage theft of the vehicle itself, its contents, or both. Car alarms work by emitting high-volume sound (usually a siren, klaxon, pre-recorded verbal warning, the vehicle`s own horn, or a combination thereof) when the conditions necessary for triggering are met, as well as by flashing some of the vehicle`s lights, and (optionally) notifying the car`s owner via a paging system and interrupting various electrical circuits necessary for the car to start.