O2 sensor bad?

[Durandal]

I know at warm idle this should be 0.5 - 0.7v. At a warm idle my O2 sensor is 0.05 - 0.84 (mostly between 0.4 - 0.8) I have been getting lower fuel mileage which leads me to think my O2 sensor was bad (don't know how old the sensor is) Would a bad O2 sensor bounce around on both sides of the desired idle voltage? Or is this more likely to be a spark plug problem? It's a jdm tt so it isn't a MAF problem.