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.

 

Also it is throwing no codes at all.

 

At 315k your cats may be plugged