This post is a 2-fer. A - to report that I found a cheap, working replacement fob on Amazon and B - to ask how to make it better.
I have a 2014 CTS-V sedan and only 1 key fob (#2). The transmitter works, but only up close. When I bought the car in '19, it worked about 50 yards away, then over the years, I had to get closer and closer for things like lock/unlock and remote start to work. I checked the battery and it's good (3v). Very rarely it says "transmitter not detected" when trying to turn the ignition but I get out, get in and it works again.
All this to say, I started looking into new fobs. After lamenting over the price of all the "OEM" fobs, I found a $27 fob on Amazon.
Keyecu Replacement Smart Remote Key Fob 5 Button for Cadillac STS CTS 2008-2013
I found the programing instructions in my manual and it worked like a charm! It programed #3. So I can now remote start, lock/unlock from a distance again. I'm going to post my instructions here, yours may vary slightly
1. Make sure the car is off
2. Remove manual/cut key from working fob and place working fob in the cupholder.
3. Insert cut key into driver door lock cylinder and turn clockwise 5 times in 5 seconds, you will hear a beep and the Driver Information Center (DIC) on the dash will say "Ready to learn electronic key"
4. Place the new fob, top down, buttons towards the front of the car into the small, curved slot at the back of the center console box, underneath the armrest lid.
5. You will hear a beep, indicating that the new fob has been programmed (it should not take more than a couple of seconds) and the DIC will report the key learned.
6. Press in on the ignition knob to exit programming.
7. Press unlock twice on the fob to finalize.
If everything worked, your new fob should do all the functions of the old one.
So there's one looming problem. The seat memory does not recognize the fob so the seat, mirrors and steering wheel don't move when I get in or out unless I press the 2 button on my door manually. Does anyone know of a way to program #3 fob to the other slots? In other words, "make the car think key 3 is key 2?"
Anyway, I hope this post helps someone in need and I hope I find the answer I'm looking for. Either way, I'm excited to report that if you need a cheap, alternative fob, the one listed above works great!