oxygen sensor question

I have a 98 Chev K1500 with the 350 vortec and I have been getting horrible mileage for the past few months. I have been asking around and never getting the same answer twice. Finally the other day my service engine light came on so i took my truck in to autozone and they plugged it in and got a bad oxygen sensor bank 1 sensor 2. They told me to replace the one after the cat so i did and my light came back on the same night. I have more than one oxygen sensor and i now understand how they work and why i have been getting bad mileage but i cant seem to find anyone who knows which sensor is bad. I hate guessing at 65 bucks a crack. Can someone please shed some light on which sensor is which or if i replaced the correct one then why I am still getting the same error code in my computer?

bank 1 sensor 2 O2 sensor is the left bank post cat sensor. as for the code, it could be a bad sensor, broken wire, bad connection, or possibly a fuse for the O2's heating element(underhood fuse block, I believe). as for the bad fuel economy, have you checked the air filter or fuel filter?

ok thats the sensor I replaced. My service engine light went off on its own but im sure the error code is still in my computer. I checked my mileage and it is back up 2mpg so its much better. As far as why my service engine light coming back and going away on its own on afer replacing hte bad o2 sensor I still dont know why it did that.
I replaced my air and fuel filter a while back so i know they are good. I'm pretty sure it was just my o2 sensor because it was reading 5 amps when its only supposed to range from .1 to .9
thankyou for your help

Replacing the Bank 1 Sensor 2 oxygen sensor shouldn't change your fuel mileage at all, all it does is monitor catalyst efficiency. The pre cat sensors are the ones that affect fuel delivery and range from .1 to .9 volts. If your light is still coming on, get it scanned again and be sure to get what code it is