97 Ford Expedition 5.4L V8 68k Miles Code P1151
Code definition of P1151, upstream Bank 2 HO2S is not switching, also indicates PCM is adjusting for overly lean condition on that bank.
So is it a lean condition on that bank, or is it a bad HO2S? Ever see the test procedure for verifying one of these? Before you go looking for a lean condition, run the test first. Look at it here.
This the rig I setup to use for all my propane needs for diagnostics, the head is something I got from NAPA, it use to have a clear hose and button for triggering, I modified it with 1/4" rubber line to a blowgun nozzle and handle with a long snout, with this I can inject propane any where I like in short bursts.
After verifying the HO2S as bad, I dissected it for you so you could see inside, the broken parts in the tip are like they were with it together, this is why this sensor could not respond to fuel changes. Remained flat during the test, would not respond to propane.