when you press the remote to open the door, the light flashes on the unit, but the door will not open. What is wrong?
This might happen because there is a problem with the torsion spring. Usually garage doors have a limit of number of time it should be opened before it stops opening. However, the person installing it will not give you this piece of information. Go to http://www.realestate.com/advice/garage-door-opener-troubleshooting-tips-93815/ for details.
There could be an obstruction.