On a "V6" engine there should be 1 O2 sensor on each exhaust manifold.
Bank 1 sensor 1
Bank 2 sensor 1
Then on OBD II vehicles(after 1995) there will be 1 O2 sensor after the catalytic converter(sometimes 2), this sensor is mainly for monitoring if the cat converter is working well enough, i.e. cleaning up the exhaust, these often last the life of the vehicle since they get "cleaner" exhaust.
Bank 1 sensor 2
The Bank 1 sensor 1 and Bank 2 sensor 1 O2's are used to fine tune lean/rich condition.
Then the average between these 2 sensors is compared to Bank 1 sensor 2 to see if cat converter is working to lower emissions, so the back O2 sensor has some effect on lean/rich but not much.
O2 sensors are rated for 80,000 miles but often work fine much longer.
Burning oil or long term "running rich" will shorten an O2 sensors life.
Since there are two O2 sensors monitoring lean/rich, if one was causing a false lean, so computer was running one Bank richer you would probably get a CEL(check engine light).
You can use a live OBD II scanner to monitor the sensors.
The MAF sensor is what tells the computer the volume and temperature of the in coming air, this sets the gross lean/rich, so would effect MPG more than O2s.
MAF sensors are easy to clean, but use MAF sensor cleaner not carb cleaner.
EGR system issues can effect MPG as well, but you would usually get a CEL if that's the case.
The returnless fuel system you have runs at 65psi of pressure, so even a small leak will cause a change in MPG.
You can rent fuel pressure testers that hook up to the fuel rail.
Start engine make sure pressure is approx. 65psi, then shut engine off and watch the pressure, it shouldn't steadily drop, it should hold.
At the end of the fuel rail is a pressure damper with a vacuum line attached, engine off, remove that vacuum line and smell it for gas smell, if you smell gas then replace damper.
Cold weather means denser air, which means more fuel is needed to get correct fuel/air mix, so lower MPG in winter is not abnormal but it is not a big change.
Also in colder climates gasoline often gets ethanol added to prevent freezing, ethanol does not have the same "energy" per gallon as gasoline, it has less, so MPG will go down, but again not much.
But combine these two things together and you may notice the difference.
Last edited by RonD; 01-06-2014 at 11:42 AM.