Replacing O2 sensors

[rnovember]

My check engine light came on awhile ago and I'm getting bad gas mileage. I don't have a code-reader or anything, but my hunch is a bad O2 sensor. Is replacing these sensors something that's easy to do yourself? I realize there's 2 or 3 of them and I'd likely have to replace them all. Or is it worthwhile to take it to the garage and have them scan it and charge me an arm-and-a leg. Any comments would be appreciated.