bad gas milage and idle

first off i ran my car for a while with a bad o2 sensor for about a month so i replaced it with a bosch oem replacment sensor ( not universal) and i adjusted my valves. the thing is before i did any of these things my gas milage was around 24-5 mpg, not too bad. now its around 20 mpg or even worse and my idel is wacky stays around 750 rpms but moves up and down about 1-200 rpm. the thing is the idle moves with the reading on my a/f guage ( FAZE brand). I isntalled all of these things at home wich is 7 hours from school on a weekend. on the way home the car was running rich the whole trip, i got about 240 miles on one tank going about 75 the whole way, and the idle was fine. now the guage reads lean on the far end and the idle is messed up. Could the o2 sensor be bad?

sorry for the long post.

Take it back and get another one. I have the Bosch O2 sensor also and haven’t had any problems with it. I have a warranty on mine.