How do I program a key fob for a 2003 Avalanche?

Close all the vehicle doors.

Insert the ignition key into the ignition lock cylinder.

Press and hold the door unlock switch.

While holding the door lock switch in the unlock position, cycle the ignition ON, OFF, ON, OFF.

Release the door unlock switch. The doors will lock and unlock to confirm the program mode.

Press and hold the LOCK button and the UNLOCK button simultaneously on one transmitter. After a delay of approximately 15 seconds, the doors will lock and unlock to confirm the programming of that transmitter.

Repeat the previous step to program up to four transmitters.

Turn the ignition switch to the RUN position in order to exit the keyless entry transmitter mode.

Operate the transmitter functions in order to verify correct system operation.

The only thing missing from these directions have to do with the numbers on the back of the fobs and the fact that once you enter the mode to reprogram the FOB all of them have to be reprogrammed. So be sure to have them all handy. As for the numbers on the back of the fob. They mean nothing. They are simply there for you to tell them appart. What ever remote is programmed first will be remote #1 and if you have seat position recall and other features tied to that driver number they will get tied to the first remote programmed. The second remote programmed will be #2. You can have up to 4 remotes programmed. I'm not sure how it works with the seat position recall with more than 2 remotes as I only have two. Also you would have to have that option on your truck.