This is needed if your garage door opener has something called "rolling
codes" (or similar to that).  It's a security feature where the code changes
and the door opener and control have to be in synch.

Josh

The instructions to program the remotes are usually written on the back
of the overhead garage door opener unit itself.

I have a unit with rolling codes and I also had to climb on a ladder to
follow the programming procedure and to press and hold the correct
button (see warning below.)  It helps to have a helper in the car while
you are performing the programming sequence.

WARNING: There are two buttons. One of them RESETS the unit itself while
the other sends the code to the remotes. Should you press the RESET by
mistake, you have to reprogram all the handheld remotes!

Working from memory here, but ...

- Point your garage door opener at the mirror, from a distance of 2 - 4
inches
- Push and hold the button on the Opener and the desired button on the
mirror
- the mirror light should start by flashing slowly, and begin flashing
quickly when it picks up the program

Note that more and more garage door openers have a frequency-hopping feature
for security, i.e., the frequency that works one time will not the next.  On
those, there is a button to push on the opener that puts it in a "training
mode" ... If you push that button, you then have 30 - 60 seconds to go
through the steps outlined above.

Good luck.