I’ve seen p1131/p1151 before I did my intake mani gasket and isolator bolts. This is for a 4.2. Either way, it definitely points to a vacuum leak in that particular cylinder bank.

Not sure why you got told “O2 sensor” and sent on your way.

 

Live data would be helpful. You can watch trims and sensor inputs. Might need to pull some plugs to see if it's running rich or correct.

If the sensor says lean the plugs should look rich because the PCM will add fuel "thinking" that there's a vacuum leak. If you really do have a vacuum leak the mixture will adjust and the plugs should look okay. Actually, I'd probably pull a plug or two anyway to start. They might tell you something.

Did you check the wires when you changed the sensor?