| The P2096 code relates to the post catalytic converter oxygen sensor and shouldn't affect the milage. I believe that it is set after the sensor reaches its operating temperature when the system runs a monitor (test) to determine if the post-cat sensor is working. The P2096 code is a lean condition and this should not result in reduced milage, unless the system enriches the mixture to correct the condition! That would be the tail wagging the dog as the pre-cat oxygen sensor is the one that should control the fuel/air mixture.
I would scan the system and see if there are any fault codes or pending codes stored Monitor the oxygen sensor output if you can and see how it's operating. It is possible that the pre-cat sensor is faulty, but there are many other faults that cause low milage. I think too many oxygen sensors are changed unnecessarily and often see the "service engine light= replace oxygen sensor" advice given without any diagnostic work. | 
