How do you reset the thief system on a 2003 Cadillac CTS?

To reset the theft system on a 2003 Cadillac CTS, insert the key into the ignition and turn it to the "On" position without starting the engine. Leave it in this position for about 10 minutes, until the security light turns off. Then, turn the key off and try starting the engine. If the security light remains on or the car doesn't start, you may need to repeat the process or consult a professional for further assistance.