I was using the A/C and then parked the car to go into a store and when I started the car to leave the store, the A/C was no longer working. It just blows warm air.
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2001 Kia Rio Question: Air Conditioner
Answer #1Bret Bodas from RepairPal Test Shop, July 13, 2009, 16:49Master
This sounds like an electrical issue. It's not uncommon for an electrical component to fail in this situation. You are driving the car, the electrical part get's very hot but works OK, then you stop and turn off the car and then the electrical component fails.
The a/c compressor may be losing the power signal due to a defective a/c relay or a/c compressor clutch may be getting power but not energizing. Have it checked out to know for sure.