They wouldn't give you the troublecode? Isn't that what they're supposed to do? Anyway, it might be P0420 Catalyst System Fault.
The reason I said gas cap is because the wife's corollas check engine light just lit up. I pulled the code and got that P0420 and a google search called up basically what the guy said, O2 sensors, then catalytic converter. Well, I noticed the gas tank was full so I checked the cap and found it was a little loose. After tightening it, I cleared the code and it hasn't come back over the past 2-3 days.