On a V engine there are two O2 sensors that could effect MPG (lean/rich)
The "upstream" O2 sensors
These are nearest the exhaust manifolds, one on each pipe
Both should be changed, they usually unscrew easily, the PITA part can be the connectors, lol, I have big fingers.
Expected upstream O2 sensor life is 80-100k, they use a chemical reaction to measure Oxygen, so do wear out, engine problems can shorten their life as well.
The other O2 sensor, single exhaust, or two O2 sensors, dual exhaust, are called "downstream" O2 sensors, these are "after" the Cat Converters, they are there to check the Cat converters efficiency so do not play a major roll in lean/rich settings.
These usually last the life of the vehicle because they get "cleaner" exhaust after Cat burns off the emissions.
Last edited by RonD; 07-22-2014 at 12:15 PM.