What to check before replacing the o2 sensors?

[fraustypath]

Autozone computers says I have going bad, is there some stuff to check before I start forking out cash? Also how tough are they to replace?

[fraustypath]

Also in price checking these sensors out I keep find large price differences. davance auto wants anywhere from 75-100 a piece, but I found some universals like these http://www.autopartswarehouse....inder

for half the price

[jsm02cpa]

Autozone told me both mine were setting off codes as well, and at $100 each, they are happy to sell me new ones. Anyone use the generic sensors and be happy with the results??

I assume that the gas mileage drops when the sensors are messed-up, so need to look into this.

[fraustypath]

gas mileage drops and you can't pass inspection from what I was told. I am going to get up under it tomorrow get a look at them to see if I can tell if one is screwed, there was some pretty bad exhaust damage that I just had fixd so that one could be bad