Yeah, "Check Engine" is a PITA. At least on Subarus, it can be something as simple as a gas cap not screwed down tight. And the dealer charges you something like $70 to hook up his computer and download the codes. On this car, I suspect it is the sensor on the catalytic converter -- hit a puddle just right, at the right point in the warm-up cycle, and the splash chills the sensor. Turn it off, and it stays off for a year.

The wonders of modern science.