2005 F150 5.4 3v O2 Sensors
 

Okay. So I was having a low power concern for several months and did some research. thought it was plugged cats. so before I spent a mint to have them replaced, I decided to just gut my exhaust from behind the cats back so I could take a peek up in there. Apparently the cats are fine, but i removed all the exhaust system behind them including the downstream O2 sensors. Come to find out, my upstream sensors were bad, so I changed them sons out with a quickness hoping to solve my low power and horrid mileage issue. After the swap, I cleared the codes and she ran like a dream...until the next time I fired her up and started driving. For some reason it's still throwing upstream sensor codes. I expect it to throw codes for the downstream ones because they're not connected but it shouldn't be throwing codes on brand new sensors. It's the same thing every time. I can clear the codes and it will run great until the next time I fire it up, then it will throw codes and default to running rough and rich. Any ideas? Tyfys in advance.

Can you provide the codes you rec'd?
The percentage of sensors being the reason for codes is not a large as one might expect.
I'm sure you know if Ford could have done away with the downstream O2 sensors, (saved a bundle) they would have. !!
Until you put everything back as it should be , sensors present and connected, there is no sense attempting to offer any other suggestions.
As much as we might like to eliminate /bypass some components , those darn computers know right away when we cheat !!
Soooo, why not go the right path and do it right , when done correctly and working correctly the manufacturer paid their engineers plenty to get it close to performing pretty good.
What do you think?