bad gas milage, black soot on the tailpipe , poor emissions, check engine light
o2 sensor should be changed at the latest 100k, sooner the better, with a much as gas is, a new sensor may pay for itself a a few weeks witht the fuel savings. rockauto.com has good prices you can even find discount codes on the web to save more money. I had a friend get 8 more mpg after he replaced his