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.
Nissan xterra doors lock themselves.
If it does its somewhere in the door.
If a door isn t fully closed or the sensor goes the car thinks you re constantly closing and opening the doors.
When you close the door because you re driving it auto locks the door.
Try and lock the doors without the key in the ignition and see what happens.
This is a new one good luck.
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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.
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.
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 you hit the lock button on the fob while any door is open the doors will lock however the horn will not chirp.
The car unlocks by itself if you leave the key in the ignition and you try and lock the doors.
If it doesnt unlock the fob is your problem.
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.
When you open the door it auto unlocks.