1. Unlock the doors with your key or keyless entry. Insert your key into the driver's side door lock cylinder and turn it counterclockwise to unlock the doors. If your Xterra has keyless entry, simply press the unlock button on your key fob.
2. Locate the anti-theft control unit (ATCU). The ATCU is a small black box located under the driver's side dash. It is usually held in place by a few screws.
3. Disconnect the power connector from the ATCU. Once you have located the ATCU, unplug the power connector from the back of the unit. This will disarm the car alarm.
4. Turn off the ignition and wait 10 seconds. After you have disconnected the power connector from the ATCU, turn off the ignition and wait 10 seconds. This will allow the alarm system to reset.
5. Reconnect the power connector to the ATCU. After 10 seconds, reconnect the power connector to the ATCU. This will re-arm the car alarm.
6. Test the car alarm. Once you have reconnected the power connector, test the car alarm by locking the doors and then trying to open them. The alarm should sound if it is working properly.
If the car alarm does not sound when you test it, you may need to replace the ATCU. You can purchase a new ATCU from your local Nissan dealer or online.
