Why is the security light on my car coming on?

Lock and unlock the drivers door with the door key. This resets the security system.

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

The Passlock system temporarily lost contact with the key

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.

Here is a link that will help with the SECURITY light.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

The gears locking up is a completely different problem. Unrelated to the security issue.

Do you have a second key. If so try using that key to start the vehicle. Dose the light go off.

If it dose switch back to the key that causes the security light to come on. If the light comes on for one key but not the other the chip in that key is failing.

If the security light dose not go out for any key you use, it is the ignition detection unit is failing and need replaced.

Looks like the TDM in the security system (VATS/PassKey) is having problems, it's typical on those cars and it can get worse till the car wont start anymore. You can find a great write-up here: http://newrockies.ca , it was the only thing that saved my Park Avenue.

honestly, any number of thing's could cause the security light to come on, i would go to my local auto store and ask them to run a coad scsnner to find the problem, good luck

disconnect the +battery post with an 8mm socket and leave the thing off for 2 minutes.reconnect it.

if our security light come on its our theft system that cause the problem most the time its the baril the chip is inside the baril