A reader recently said that they bought and programmed a new remote for their vehicle and that once this was done, their original remote no longer worked. The reason for this is that many systems automatically erase the programming of all prior remotes when you enter programming mode and program a new remote. This is so that your security can be maintained if a remote is lost or stolen.
For systems with this “erasure” feature, you should have all of the remotes you plan to use with the vehicle present at the time of programming. The programming instructions will explain how to program multiple transmitters.