2006 Pontiac G6 Check Engine light

I just put a BMW style cat back exhaust on my 2006 G6 GTP. Ever since then, I get a trouble code saying "System too lean (Bank 2)". I either disconnect my battery or have the code cleared and the light stays away for roughly a week. When the light does come on, my onboard gas mileage computer says my mileage is better than when the light is off. The car will backfire somewhat during deceleration. Could this problem be a lack of proper backpressure or a possible O2 sensor, or something else I'm overlooking?