I've searched P0161, and now I know what it means, but could it cause the car to run like crap?

It seems to depend on which thread you read on weather or not it can affect the car at all. My symptoms are like a misfire. When I shift into 4th (non-WOT), it kicks me a bit.

I recently installed Mac headers w/ cats in case it matters. Any help would be appreciated. Thanks guys.

Oh yeah, and I didn't get the code till almost 2 weeks after the install and tune. Ran great until then.