1998 Chevy S-10 O2 sensor problems

My O2 sensor flipped a code about 3 weeks ago that said low voltage, lean/rich mixture. I went and replace the O2 sensor thinking that it had died but a day later the code came back. My gas mileage has also been going to hell since the light has come on. Anything else that could be wrong it? I poured some injector cleaner in the tank that we had lying around the house so I dont think its a dirty injector. I want my 30 mpg back, help?