desert whomper

Ok, a while back my CEL light came on, ive only had the truck since the 4th day of september. The CEL came on the first time a while back after a long drive to the willow springs race track. So i checked the light and it said bad 02 sensor so i researched this a bit on the net. I had the the sensor before the cat and after a week i replaced it with an OEM denso sensor, plugged right in. When i was working on my tranny now last week (new throw out, pilot bearing and flywheel redone) i noticed there was 2 02 sensors, im thinking crap how could i have missed that, after all the looking around i did too. I was hoping that the one i did replace was the bad one. Now last night i was on another long drive and the light came on, im think dang this sucks i bet its the 02 sensor. My truck does run a lil rich and i have checked everything. Tomarrow morning i will pull the code up. I usally keep parts for my truck for a long time but this time i didnt because i didnt want crap laying around , man im just mad at myself but live and learn.