AC question

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okay usually at time, i cruise with the windows down and have my ac OFF with the option of air coming from the outside so i can also get some breeze thru my vents. I have my knob all the way to the left for cool. Just recently, when i cruise like that, i feel WARM/hottish air coming from my vents even though its not even hot outside and I have the knob all the way left. Now the only way i found to fix this is to turn my AC ON for just a few seconds to feel the cold air, then i turn off my ac