After installing all new and all 4 o2 sensors, i'm still running 0134 and 0154, no activity bank1 sens1, bank2, sens1. (both front o2's)

I had first replaced all 4 o2 sensors with new ones from Atlantic British...then noticed the next week that I was missing almost all of my studs where the exhaust manifold and y-pipe meet. There's supposed to be 3 studs and nuts on each side, I was down to 3 studs and 2 nuts total. I also had my passenger side post-cat sensor come unhooked and get ripped out by the driveshaft, thats another notch on my learning belt!

Anyways, I went with another new o2 sensor from BP of Utah and all post-cat codes went away (3 of them) at once immediately afterwards.

Question: if my new o2's were in for a week before I fixed the gap from missing studs and a mis-aligned gasket...could that have killed my front 2 sensors somehow?

Any other possibilities for why I might still be running these codes if the sensors are fine?

I'm still getting crappy gas mileage at 200 miles per tank, and I've recently had a head gasket job with new plugs (plats) and wires (8mm Mags), t-stat, etc. Always run 93 oct.

What am I missing here?