Severe idle problems after TB/Plenum/O/R X-pipe install

I think I may have found the possible problem..

I went and took my plenum and TB off as a last resort, to check every gasket/seal again.

My EGR gasket is pretty pitted up, and there was carbon all over the spot on my plenum where the EGR connects to the plenum (where there's a gasket). There is no reason for carbon to be all over unless that gasket was leaking... At least that's the way I look at it. If it wasn't carbon, it was parts of the gasket coming off... Either way, there was **** on there where **** shouldn't be.

It doesn't look bad enough to create a constant leak, just every now and then maybe... which would support the symptoms I'm having.


Also, I looked on my old plenum where the EGR goes... A very very thin layer of like 1/2 of the gasket was still left on the old plenum.

So, I'm really thinking this may be it... I'm going to pick up an EGR gasket and IAC gasket (even though my IAC looked ok) when autozone opens in 1 hr, and if that doesn't solve it, next step is the o2 sensors (because I pretty much just eliminated all possibilities of vacuum leaks).

Does anyone think this could have been causing it?

I started the car after putting the new gaskets in and resetting the computer, and it idled a little weird at first (just a rough idle, no jumping or major fluctuations), but that sorted itself out within 10 minutes of idling.

I drove the car for 50 miles with no CEL or problems. In the past, the CEL has never taken more than 30 miles to come on. I will be driving another 50 later today probably, so I'll see what happens.

Well, around 60 miles, my CEL just came back on. I had one slight problem with the car (car hesitated when I first started it up and took off, was fine after that).

However, I'm only throwing one code now, 1151. I must be making progress...
When I got my codes pulled at Autozone a while back, they said P1151 was "accelerator position sensor property malfunction", but everything I've found online says it is "Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2".

I'm thinking I had a leak at a gasket AND maybe a bad o2 sensor. That would most definitely make it run like ****.

So, I guess I will be changing driver's side upstream O2 tomorrow.

How hard are the upstream o2s to change? Are the harnesses hard to get to?