Ok, I have a 1998 F-150 with a 5.4 4WD. I currently have 315k on the odometer (original engine, trans, and tranfer). I had noticed VERY poor economy and decided to dig into what could be the cause so I hooked up the scantool. Both left O2 sensors were looking great. Good fluctuation and heated up quickly. Now the right bank upstream was at .1699 upstream and the same downstream. I checked it a little later and the upstream changed to .0. I replaced the rear O2 and now I'm only getting ..035 to .045.... I threw in a set of O2 sims from a 03 Mustang I had laying around with the same result. Thoughts? Also, the truck is definitely under powered for where I feel it should be.

 

So...I translate as the engine is running so poorly that the cat converters have not been able to decon the exhaust flow with much effectiveness, hence the lack of difference between front and rear O2 readings.

You do a compression check yet? That's quite a long life that engine has lived. Might have a few dead cylinders.