If it doesnt unlock the fob is your problem.
Nissan xterra doors lock themselves.
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.
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.
Also if you get some water into the lock switch on the door that can cause it to malfunction as well.
Try and lock the doors without the key in the ignition and see what happens.
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If it does its somewhere in the door.
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.
I had that exact issue a couple weeks back when i closed the passenger seatbelt in the door.
When you open the door it auto unlocks.
The car unlocks by itself if you leave the key in the ignition and you try and lock the doors.
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.
Whether it is in the fob or the lock itself is what you need to figure out.
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.
All doors will lock automatically within 5 minutes unless one of the following operations is performed.