[ruddneck]

Update: I took the car back to a dealership where the service manager told me to expect to leave it for several days while the mechanics drove it until the check engine light came back on. Because the Prizm computer is so small, he said, the best diagnostic info is obtained as soon as the light goes on.

However, I heard back from the service team leader an hour or two after I dropped the car off. As with other folks' experience with this problem, it was the mass air flow sensor. (I had to bite my tongue when he said that after I mentioned it beforehand!) Parts and labor and a new sensor cost me $352.99 overall. Pricey, but the car runs like new now, and even the A/C seems to cool quicker (though I reckon I have to wait til summer to test that out fully).