Guys, I've been doing a lot of reading on some tuning forums.

One thing I see over and over again is that exhaust leaks cause inaccurate 02 readings. Even very small leaks! The result is often excess fuel consumption and one bank running richer than the other.

So.. if you feel your mpg is down as compaired to where "it used to be"? A thing to check is for exhaust leaks. **especially** at the header to mid-pipe connection. Leaks back by the muffler can effect it.. but not much.

Also, 02 sensors that have more than 60k miles on them will start to get "lazy". Meaning that they work. But they swing from high to low and start to falsly read leaner and leaner which adds fuel.

 

True!

The "Donut" gasket was leaking on my truck (the gasket before the cat).
My mileage was no good, even though the leak was very small. I repaired that leak, replaced the upstream sensor, and the mileage improved.

It also fixed a intermittent CEL.
(On of those CEL's that drive you nuts because it's so vague!)

 

Any exhaust leak before the sensor will cause lean readings and richer mixtures