Hop in and drive like nothing happened. Call the shop right away to tell them what happened. If you wait a day or two, they can rightfully assume you did something long after leaving their shop. When you call right away they will know it was their mistake and they deserve the chance to make it right. Any reputable shop will do that to keep you happy. Please don't be angry with them. Doctors make mistakes too, but mechanics are held to much higher standards.
The coolant is half water and half antifreeze. Adding a half gallon of straight water means there's a quart of water in there that is supposed to be antifreeze, and that is not nearly enough to lose sleep over. Your mechanic will check the freeze point which I'm guessing will probably be around minus ten degrees Fahrenheit instead of minus 35 degrees. Even if it's minus fifteen by you, it won't freeze solid. The coolant will just turn to slush. If the reservoir is empty now, leave it that way. Once everything else is checked over, your mechanic can add straight antifreeze in the reservoir to replace what was lost. It will get drawn in and out of the cooling system each time you warm up the engine, and will be sufficiently mixed within a few days. After that, stop back at the shop at your convenience to have the freeze point checked once more.
Friday, March 6th, 2015 AT 12:17 AM