Garage door openers come with a couple of remote controls that can be used to open and close the garage door from within your vehicle. A code is preset and will open the first time you use it. For security reasons the homeowner should reset the remote opener. This is also true for new homeowners who are given an already working door opener with the home.
Liftmaster is one of the more popular automatic garage door openers and is easy for homeowners to reset on their own. The following is a short video walking you through the rest process for your Liftmaster garage door opener.
How to program a Garage Door Remote & Change Battery