My ac will blow cold air one day and the next day it will not work.

Unfortunately, 'IF' the problem is NOT present at the time of testing and cannot be duplicated, NO one can diagnose it!! Intermittent failures are near impossible to find.... unless there is a common solution for the frequent occurrence of that particular vehicles/problems! ... However, some repair facilities try harder than others!! One would think a Dealer should have the latest & greatest solutions.... Maybe find an independent mechanic and let him/her drive it for a couple days.... Sorry but that's about all we can do for you,,,, 'online'.