Antitheft system keep goes on so i can't start my vehicle

This is the dreaded antitheft security system. This a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other.
The "disabled icon" illuminates when that signal gets broken, or not seen at all. When the communication breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. (like it thinks it's being hot wired) The car shuts down because it is not getting that signal from YOUR specific key. This is why the icon turns on and shuts down your vehicle. It will either turn off the starter, or the fuel injectors. The vehicle may run for 2-3 seconds before shutting down. This link explains it pretty well.
http://www.diy-gm-security.com/

This may be related to the antitheft system The system is known for turning off the starter and/or injectors when it fails. Is there a light on the dash that says SECURITY? This is a common problem worth looking into. http://www.diy-gm-security.com/

The security, antitheft or lock picture/icon is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. This link explains it pretty well.
http://www.diy-gm-security.com/ NEWROCKIES inc

Use the remote to deactivate alarm or use key in the door lock to unlock the door. The problem you had is when you lock the door and the alarm activates it must see a disarm signal or else it thinks someone broke the window and used the switch to open the door.

The security, antitheft or lock picture/icon is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM (Theft Deterrent Module). These 3 parts are in CONSTANT communication. If the key is in the ignition, then these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car misinterprets this as an attempt at theft and goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. This is why the security light turns on and shuts down your vehicle. This link explains it pretty well.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

Gm vehicles are known for failing antitheft systems. One of the symptoms of this failed system is that the vehicle will start for 1-5 seconds and then cut the power to the fuel injectors and stall. This is the antitheft prevention. It's purpose is to keep people from "hotwiring" the vehicle" Normally this will be followed by a light on the dash or gauges that says SECURITY or ANTITHEFT. if you see this light, it is pretty much a dead give away of the problems. But since the security system controls the light itself, it may not always be present. This link explains the problem and offers solutions. Just a thought, hope you get it all figured out.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

If it's the antitheft security system keeping you from starting your vehicle. This link will help.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

I have no idea on the horn problem. Maybe loose wires in the wheel or steering column. The SECURITY light is another problem all together. Here is some info on the security.

The security light is a visual indication that something in the car's antitheft system is failing. In most GM cars and trucks there is a hidden system that most people are unaware even exists. From the moment you put your key in the ignition, there begins a constant line of communication between your key's security chip or transponder, to the ignition's lock cylinder, and then to the car's TDM. These 3 parts are in CONSTANT communication. If the key is in the ignition, these parts are sending signals to each other. The security light illuminates when that signal gets broken, even just for a second. When the signal breaks, the car goes into antitheft mode. The car thinks its being stolen or hotwired because it is not getting that signal from YOUR specific key. There is where the security light turns on and shuts down your vehicle.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

96 nissan pathfinder antitheft keeping car from starting keys dont work in door anymore

Hook up the battery, then put your key in the door lock and lock and unlock the door. That should cancel the antitheft. If it doesnt work on the drivers side move to the pass. side door.