HelloCustomer here is the reprogram procedure for the alarm.Try it and let me know if it works.
Try this procedure if it doesn't work you will need to have the codes read with a GM scan tool caalled a Tech 2.Before you do the procedure remove both battery cables and hold them together for 30 seconds and have a friend hold the brake pedal down while you are doing it.Reconnect the battery cables and perform the procedure.Hooking up a backup battery or another car running to keep your battery charged would be a good idea during the procedure since it will take 30 minutes or more.
Use this procedure after replacing the following components:
The passkey III (PK3) keys
The theft deterrent control module
The PCM, see important above
This procedure is not available on vehicles equipped with option code (Z49 or BAE).
If replacing a theft deterrent control module with an GM SPO Replacement Part, perform the procedure to setup a new theft deterrent control module prior to the 30-minute relearn procedure.
With a master passkey III key (Black), turn ON the ignition, with the engine OFF.
Observe the Security light, after approximately 10 minutes the light will turn off.
Turn OFF the ignition, and wait 5 seconds.
Repeat steps 1-3 two more times for a total of 3 cycles or 30 minutes.
The vehicle learns the key transponder information and/or passwords on the ignition switch transition from OFF to CRANK. You must turn the ignition OFF before attempting to start the vehicle.
With a master passkey III key, start the vehicle. The vehicle has now learned the key transponder information and the PCM has now learned the fuel continue password.
With a scan tool, clear any DTCs.