If your car has a remote key fob, press the "unlock" button to turn off the alarm. This will usually silence the alarm and disarm the security system.
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Turn Key In Ignition
Inserting the key in the ignition and turning it to the "on" position can sometimes turn off the alarm.
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Disconnect Battery
Locate the car battery and use a wrench to loosen and remove the negative terminal. This will cut power to the alarm system and should silence the alarm.