What would I tinker with, I had my O2 sensor replaced when I first bought it, but where I bought it from the mech. there I don't think he'* very good, he'* foren. Also how many O2 sensors does my car have cause the bill for parts was like 60 some dollars. I had it fixed there cause he did it for free, from my check enging light.
With a bad O2, you won't necessarily get a SES light. The O2'* are cheaper thru www.gmpartsdirect.com. I think they're under $50.
O2 sensors just get tired over time, and cause you to run a bit rich. That'* where the mileage goes. I'm not saying this is your problem, but I AM saying this is one of the first things you need to look at if you know your sensor(*) are older than 50k or so.