When you open the door it auto unlocks.
Nissan xterra doors lock themselves.
If you want to change it get a new alarm if not be sure that your insurance company knows of the auto arming anti theft system and it will save you money at least with usaa insurance.
I had that exact issue a couple weeks back when i closed the passenger seatbelt in the door.
Whether it is in the fob or the lock itself is what you need to figure out.
All doors will lock automatically within 5 minutes unless one of the following operations is performed.
The other thing that can happen is if you hit the unlock key after the doors are closed and don t open a door again with in 60 seconds the doors will lock.
Try and lock the doors without the key in the ignition and see what happens.
Any door is opened a key is inserted into the ignition switch and the key is turned from off to on i wasn t opening the door.
This is a new one good luck.
If you hit the lock button on the fob while any door is open the doors will lock however the horn will not chirp.
If a door isn t fully closed or the sensor goes the car thinks you re constantly closing and opening the doors.
The alarm self arms therefore after a certain amount of time leaving the truck unlocked it will lock all the doors and arm the alarm system.
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When you close the door because you re driving it auto locks the door.
I would take the battery out of the fob lock the door with the rocker switch on the inside of the door itself then close it and see what happens.
If it does its somewhere in the door.