Do you run your A/C on recirculate all the time? If so you probably have some mold/mildew in the HVAC box. They sell a spray that you can spray in the inlet vents (outside on the cowl) or you can use Lysol. Recirculation should only be used to cool car down, then once you are comfortable you set it to normal A/C known as normal or economy in the past.
that is normal with auto climate control. at first, or in rare conditions, when the temp quickly creeps up faster then the auto climate can keep up it will switch to recirculate mode for maximum cooling. Once the set temp is reached it should revert back to normal.
Non auto climate control systems should never go to recirculate on their own.
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