Twice I have gone to oregon from the Seattle area with my HHR. While there I fill up for the return trip home. 150 - 200 miles later I get a Check engine light and set the code P1174. Resetting it makes it go away until I go there again and the same thing happens. I am not going to get excited about it until I get it again without going to Oregon. Just curious.
My thought is that I may have an oxygen sensor that is nearly out of spec. The Oregon gas is most likely blended differently and that is just enough to set the light. I have not connected to the computer yet to check all the O2 sensors.