Joane, could this be the discharge for a sump pump?
Or, depending on if you are lucky enough to be where you're running your A/C right now, it could be the condensate drain for your A/C.
Also could be a drain for a built-in humidifier.
If none of these ring a bell, post again with a little more info;
-how big is the pipe?
-is your furnace in the basement or on ground floor?
-does this pipe come from an area of wall that encloses the house, or is it actually within the garage area?
- does it point straight out of the sidewall roughly a foot or so from the ground, or does it exit the wall at sidewalk level or below?
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