you have changed the batteries and they are good ones? Have you added any new appliances in the garage area that could cause some interferences?
Have you done anything different in the garage area that could effect this bad reading?
Are your codes constant or on a revolving wheel?
These are very general questions, but I am just drawing straws.
Four years is not old and is not actually new. I would hesitate to spend too much money on any sort of repairs unless its not too expensive. These things are so inexpensive now days, it could be better to get a new unit.