A car alarm is an electronic device installed in a vehicle. The alarm’s purpose is to discourage theft of the vehicle and its contents. Car alarms work by emitting a loud sound and flashing lights when a sensor is activated. Also, the alarm may notifying the car’s owner via a paging system and interrupting various electrical circuits necessary for the car to start.

Car alarms should not be confused with immobilizers. While the purpose of both may be to deter car theft, they operate in a dissimilar fashion. An immobilizer generally will not offer any audible or visual theft deterrence. Immobilizers do not require any input from the driver.

Alarms come with a mix of features. Remote car alarms typically consist of an additional radio receiver that allows the owner to wirelessly control the alarm from a key fob. Vehicle alarms typically come equipped with an array of sensors along with immobilizers.

Most OEM alarms are typically armed and disarmed with the vehicle’s keyless entry remote. On many vehicles the key cylinders in the driver or front passenger door activate switches. Therefore, when a key is used in the door the alarm will arm or disarm. Some vehicles will arm when the power door lock switch is pressed with the driver’s door open, and the door is subsequently closed. Some vehicles will disarm if the ignition is turned on. When the vehicle is equipped with a key-based immobilizer and an alarm, the combination of the valid key code and the ignition disarms the system.