I have a 2008 Mazda5 that is running fine. Today when I went out to eat the "AT" light on the right side of the dash came on. I had 2 different O'reilly auto parts store check for any codes but nothing showed up. She runs fine, no skipping or anything. Any ideas out there what the "AT" might be about?

 

I would take your car to Mazda for a code check. Aftermarket scan tools don't have the full scope of codes & diagnostic of the genuine Mazda tester.