The 2003 Cadillac CTS came standard with keyless entry, a panic alarm, and a Passkey III engine immobilizer system. The immobilizer system is designed to detect whether or not a correct key is being used to start the engine. If you -- or a potential thief -- attempts to use a key that isn't programmed to the vehicle, the powertrain control module cuts the fuel supply and the engine stalls. If the VIN on your CTS starts with a "1" or a "4," you can program a new key yourself, but if the VIN starts with a "2" or "3" the key must be programmed by a dealer or locksmith.
New Lock Cylinder and New Key
Insert a new key into the ignition and turn it to the "On" position. Observe the anti-theft light -- It will illuminate and stay illuminated. Wait for about 10 minutes, or until the theft light turns off, and turn the key to the "Off" position. Turn the key back to the "On" position within 45 seconds. Wait for another 10 minutes, or until the theft light goes out, then turn it off again. Repeat the key cycle one more time. After the theft light turns off the third time, turn the key on one last time. Start the engine to ensure you have successfully programmed the key.
Adding a Spare Key
To program an extra key to the vehicle, insert an existing key into the ignition. Turn the ignition on for five seconds,and then turn it back off. Remove the key. Within five seconds, insert the key you wish to program, and turn on the ignition on. After five seconds, turn the ignition off and then on again. You have now programmed your spare key.