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Prius Alarm Passive Mode Change Procedure
This procedure sets the alarm system to automatically arm. After you turn the ignition off, remove your key, open and close the driver's door, the alarm will automatically arm after 30 seconds. This will not lock the doors, only set the alarm. Notice about half way down on the right side of the instructions it says to "Close and open the driver's door 3, 4 or 5 times" without any explanation. Well, 3 times and the alarm goes off immediately when a door is open, 4 times gives you a 15 second delay and 5 times gives you a 30 second delay. In other words, if you open/close the door 4 times at the appropriate time in this procedure, the alarm will set 30 seconds after you exit the car, then when you get in (with the alarm set), you have 15 seconds to put your key in the ignition and turn it to the ON position (or start the car).
(Toyota Repair Manual, Volume 2, Page BE-74)