My antitheft is in theft mode and won't let the car start.how do i know if the antitjeft device is keeping it from starting?can i bypass the antitheft device?

I'm afraid you are out of luck. The anti-theft is part of the instrument cluster and the body computer. Its almost impossible to bypass it.

simply bypass the anti theft device with a inline resistor go to 03 buick bypass antitheft device

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 (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 misinterperates 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/

The security, antitheft, or passlock 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 (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 misinterperates 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 may help you find ways to bypass this or fix it yourself.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

The security, antitheft, or passlock 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 (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 misinterperates 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 may help you find ways to bypass this or fix it yourself.
http://vatspasslockpasskeysecurityhelp.yolasite.com/

yes

try with the antitheft bypass.
the problem seems to be the door switches which are not working giving fasle alarm to anti theft device. use contact cleaners on the door switch with the cabin light active on door open. you may then evaluate wether working or not. check all the doors one by one

Your car starts and dies due to the failing antitheft system. Your vehicle is not receiving the "fuel enable" signal that it needs to keep running. So the injectors get turned to off to prevent theft. You will need to diagnose which part of the antitheft system is failing. Or bypass the entire security system. This link will help you understand the problem, and depending on the year of your vehicle, possibly a good bypass solution.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

does the motor crank when key is turned?If so check for a spark at the plugs.Get an auto electrician bypass your alarm

If the security light is on, then you have a failed antitheft system. This light is only for this. If you bypass the system, your car will work perfectly again.
http://vatspasslockpasskeysecurityhelp.yolasite.com/