Try this it worked on my 2003 Alero
Up to 4 transmitters may be programmed.
Transmitter Programming Procedure (Method 1 - No Tools Required)
To program the transmitters, perform the following procedure while seated in the vehicle driver's seat:
Close all the vehicle doors.
Remove the ignition key from the ignition lock cylinder.
Press and hold unlock on the door lock switch.
Important
To successfully enter the keyless entry programming/diagnostic mode, steps 4 through 6 must be completed within 25 seconds of pressing the door unlock switch.
While holding the door lock switch in the unlock position, insert and remove the ignition key twice. Do not rotate the ignition lock cylinder.
Insert the ignition key a third time leaving the key in the ignition lock cylinder. The ignition must remain in the LOCK position for the rest of the procedure.
Release the door lock switch. The chime will sound three chimes to signal that transmitter programming/diagnostic mode is active.
Press and hold the lock and unlock buttons at the same time on the keyless entry transmitter for approximately 4 seconds. The chime will sound two times to signal that the transmitter has been programmed successfully.
Repeat step 7 for the rest of the transmitters (for a total of 4 transmitters).
To exit the transmitter programming/diagnostic mode perform either of the following:
Turn the ignition switch to the RUN position.
Remove the ignition key from the ignition switch.
