I had this problem on both the 5.3L in my truck and the 5.7L in my Caddy. Turns out the upstream sensor was getting worn and causing the problem. The upstream ones gets a lot more heat and wears faster than the downstream. So basically you put a sensor upfront that was in better shape. The worn one moving downstream still works good enough. I'd recommend you replace the one you just moved down. Put the new one upstream and the orignal downstream back. If you don't replace the weak one you're going to see errors coming back later. Also keep in mind that if the error comes back after the new sensor then you might have a damaged cat. Especially if you've been running rich for a while.

 

I swapped them from the passenger side to the drivers side.
I was hoping the code would change to bank 1, but now I get no code.
Strange, but that's how computers are.
We'll just run it like that for a while, and see what happens.

 

I just remembered something.
The guy installed a K&N air filter.
I wonder if oil got to the MAF????
I'll clean it and see how dirty it is.

 

That's not a bad idea. I use K&N filters and they work really well. The only problems I've seen with them is when people get carried away oiling them. They take a light amount of oil and need to sit for a while for the gauze to absob it. Good luck with the cleaning.

One other thought. If you have been running really rich then the cats and o2 sensors are dirty. Try a can of Cataclean or similar catalytic converter cleaner. I did this and got another year out of the cat in my other car. It does clean the sytem pretty good.

 

Was running fine, until it started to be hard to start.
Still ran fine.
Didn't have any codes come up.
I started checking the system and everything checked out good.......except.
Fuel pressure wouldn't hold.
Did a on-car injector clean with no luck.
On the test drive a code finally came on.
The three codes listed.
The MAF and TP could have been when I disconnected and didn't erase them while doing the testing.
Erased the codes and did another road test.
The PO175 came on fast this time.
Found two injectors leaking, and took a chance with a whole set of new "made in china" ones.
No more leaking injector.
Pressure holding.
Went on a drive and OK.
Few days later code came back again.
Pressure still holding.
Swapped the upstream O2 from side to side.
Now no code.
But a slight shudder at around 3000 rpm.
Runs fine otherwise.
So that's where we are now.