O2 sensors and gas mileage
 

Bought a 2002 Concorde Limited recently. It was showing codes for all four O2 sensors and, of course, the CEL. Investigation showed that someone had replaced all four sensors previously with Bosch units and reading and asking questions on this forum indicated that Bosch will not work in Mopars. The car was running "ok" with the Bosch units, the main symptom being the CEL and codes, and an occasional stalling after a warm restart, ie: in a parking lot.
The car was getting between 19 and 21 mpg in town on the OTIS with the Bosch sensors in place.
I changed all four sensors to the recommended NTK units and the car now runs perfectly. The codes self corrected, the CEL went out, the stalling stopped, and it seems to run about a half a mark cooler on the temp gauge. The car acts exactly as it should, EXCEPT, now it gets 15 to 16 mpg in town on the OTIS.
What did the change in O2 sensors do to change the mpg so dramatically, especially in light of all the other corrections that occurred with the change?

While there are many other factors usually new O2 sensors improve fuel mileage. Obviously driving habits, plugs, air filter, tire pressure, etc are part of the equation.
I would suspect the Bosch sensors were causing lean fuel mixture calculations and the better fuel mileage.
PCM may take some time to adjust fuel trims to the new correct sensors.
"Scanner Danner" on YouTube has some great videos on the topic

Thanks very much for the reply. Will run a couple of tanks thru it and see if the fuel trims change any. At least it is running with no codes or CEL, a first since I got it.