Here is the Service Manual procedure for relearning the PassLock:
Programming Theft Deterrent System Components
Programming Replacement Passlock™ Sensor, BCM, or PCM
Due to component variability, the vehicle theft deterrent (VTD) system must have the learn procedure performed regardless, if the vehicle starts on the first ignition cycle after a VTD repair.
All codes in the theft deterrent module must be cleared for a relearn.
The body control module (BCM) must be programmed with the proper RPO configurations before performing learn procedures. Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control System.
If replacing the BCM with a GM Service Parts Operations (SPO) replacement part, the module will learn passlock™ sensor data code immediately. The existing PCM however, must learn the new fuel continue password when the BCM is replaced.
If replacing a PCM with a GM Service Parts Operations (SPO) replacement part, after programming, these modules will learn the incoming fuel continue password immediately upon receipt of a password message. Once a password message is received, and a password is learned, a learn procedure must be performed to change this password again. A PCM which has been previously installed in another vehicle will have learned the other vehicle's fuel continue password and will require a learn procedure after programming to learn the current vehicle's password.
Use this procedure after replacing any of the following:
10 Minute Learn Procedure
1. Turn ON the ignition, with the engine OFF.
2. Attempt to start the engine, then release the key to ON, vehicle will not start.
3. Observe the SECURITY telltale, after approximately 10 minutes the telltale will turn OFF.
4. Turn OFF the ignition, and wait 5 seconds.
5. The vehicle is now ready to relearn the Passlock™ Sensor Data Code and/or passwords on the next ignition switch transition from OFF to CRANK.
The vehicle learns the Passlock™ Sensor Data Code and/or password on the next ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
6. Start the engine, the vehicle has now learned the Passlock™ Sensor Data Code and/or password.
7. With a scan tool, clear any DTCs if needed, history DTCs will self clear after 100 ignition cycles.