Any adjustments it's doing shouldn't be big enough to cause a check engine light. If I were to guess it sounds like it would either be failing cats, or an oxygen sensor.

 

The CEL code was P0430 - catalytic converter performing out of tolerance. However the light has been off for a couple of weeks, and the car easily passed the emissions test here in Colorado. I changed gasoline brands to Shell and changed my driving habits to use lower gears and push the car harder. Of course the mileage suffers, but that's a small price to pay.