Poor Mileage - 97 Sierra 4.3

I have a 1997 Sierra with the 4.3L V6. Recently I had a problem with it not starting, http://www.automotiveforums.com/vbulletin/showthread.php?t=664144, which I think is probably related to the cap/rotor. After that I got a MIL and it gave me an O2 sensor code. After a couple of days the MIL went off.

The light has since come back on and remained on. I am now getting really bad gas mileage. I haven't had a chance to get it in to have the code read, but I'm suspecting the O2 sensor again.

Would a bad O2 sensor cause poor mileage? Other than the mileage, the truck seems to be running fine. It does smell a little rich, but I'd expect that with the mileage I'm currently getting.

The o2 sensor tells the ecm when to make the fuel rich or lean. If the o2 sensor is costantly reading lean then it would cause the computer to tell the injectors to dump more fuel to compensate for the lean code. When was the last time you replaced the 02 sensor?

The O2 sensors have never been replaced.