Security light comes on and vehicle wont start

need to boost ,charge and or replace battery if problem holding charge have vehicle boosted to when their is enough power to start vehicle that should also be enough power for the security system to recognize the key

check that it has power at the coil ,crank sensor,etc. with a test light when cranking it over.

You lost power to the security module and it now needs a "relearn" performed. Make sure the battery stays connected for this relearn.

Turn the ignition to the "on" position and verify that the theft light is on or blinking. Then leave the key on for at least 30 minutes without disturbing the vehicle. After 30 minutes come back to vehicle and observe the theft light. Should be on steady or off. Do not turn the key off, move it from "run" to "start" and attempt to start vehicle. Engine should start and run if system was successfully relearned.

This may be due to the security system failing. It is very common on GM vehicles and hard to diagnose without the security light coming on to let you know. Look at this website, may help you out http://vatspasslockpasskeysecurityhelp.yolasite.com/

When the light is on, the security feature of immobilizing car-wont allow starting-is active. Pretty difficult and time-consuming to permanently disable the security system, but some have figured it out and published their research over the internet. If yours gets worse, if the light stays on more frequently, you may want it permanently removed. I think it was a ruse by GM and car makers to add cost, and cause more frequent repair visits, but that just me.
Google "disable vehicle security system" or some such, to see results.

could be a security relean.
•Insure that the battery is fully charged.
•Turn the ignition switch from the OFF position to the CRANK position attempting to start the vehicle. The vehicle will start and then stall.
•Leave the ignition switch in the ON position while observing the security indicator.
•When the security indicator turns off, which can take up to 10 minutes, turn the ignition switch off. Wait 10 seconds.
The BCM and the PCM will learn the new code on the next start attempt.

to disable the security, make sure all doors are closed, and the key out of the ignition, go to the passengers side lock and unlock it a couple of times with the key, do the same to the drivers side.. most of the time this will disable any security that the vehicle has got...also just saw last week, a family member of mine went without a vehicle 3 days because it wouldnt start and had a security light on...i went and looked at it.. used the same key he always used, i noticed that it was a key that was duplicated, and not the original.. i had him get me the original, and the vehicle started right up... never done that to him before, not sure why it started doing it.. but the key had a chip in it that the ignition reads, and if the key isnt the factory chiped key the vehicle wont start.. dont know if your vehicle is the same, but its something to think about.. hope this helps

It may be as simple as trying a different key but the security systems on gm vehicles have had problems with the ignition cylinders going bad and preventing the car from starting.

Leave the key in the on position for 20 to 30 minutes without trying to start the vehicle. In most cases a lose of power to the computer has happened and the security must be reset.

You can beat this yourself. This link explains why the problem exists, and how to fix it yourself.

http://vatspasslockpasskeysecurityhelp.yolasite.com/