If you need to reset the anti-theft system on your GMC vehicle, follow the steps below. Using the GM key or keyless remote, unlock the door and start the engine. Let the door of the car open after you leave the car Wait until the light of security stops blinking before using a typical key. Once the door is closed, press the Lock button on the keyless remote. As a result, the theft-deterrent alarm is also activated.  Within 10 seconds of not opening the door or pressing the unlock button on the keyless remote, the theft-system will be activated. There are several ways to fix anti theft system car won't start problems. There is a good chance that at least one of these tricks will work on most cars.