Maybe a vac leak somewhere has you running a bit lean? Could always be a bad sensor. If you have tons of time, you can swap the two pre-cat sensors (driver and passenger). If/when the code comes back, you'll know whether it's an actual lean condition or just a bad sensor, depending on what bank it pops back on....
Something I forgot to mention in my previous post is that I just did an engine change and was not getting any codes prior to the engine change. I do have a relatively new O2 sensor that is on my old engine (It was replaced 1 year ago). I may give that a try.
I didn't get any other messages with the 1151, so I didn't think it was the O2 sensor itself. I put all of the vacuum lines back on and doubt there are any leaks, but anything is possible.
I initially thought this was coming from the passenger side for some reason. I wanted to verify it wasn't an exhaust leak so I tightened it up a bit more. I will try the same on the drivers side to verify that isn't the problem.
Will I get a single P1151 if the O2 sensor is bad? Maybe this is just the begining stages of it?