The electronics controlling the anti-theft security alarm system built into a Toyota Sienna minivan can malfunction and refuse to arm/disarm as it must. A procedure must be followed exactly in order to reset the security alarm system on the Sienna so that the anti-theft security alarm will once again work properly. You can do this yourself -- there is no need to take the vehicle into a service center.
Sit in the driver's seat and insert the ignition key in the ignition slot. Rotate the key a quarter turn clockwise.
Run your hand under the dashboard beneath the steering column until you find the Sienna's reset button. Press the button and hold it in until the alarm light on the dashboard glows a solid color instead of blinking and the horns blare once. The Sienna system has now been reactivated.
Rotate the key back to its initial position. Rotate the key a quarter turn clockwise again. Press and hold the reset button again. Release the button once the LED glows and the horn sounds. The Sienna anti-theft alarm system is now activated. Rotate the key back to its original position, remove it and exit the vehicle.