I wanted to know if it is unheard of for the brand of gas you put in your car to cause an O2 Sensor to run lean. I ask because my 2005 Taurus had this code and a few other related codes a couple of months ago. Using some of the advice on here I replaced the vacuum "O" ring and flushed out my fuel system using Chevron gas treatment and also C-foam. After a couple of times the lean code appeared to have corrected itself. Since then I was only putting chevron gas in my car. Well yesterday my car was about 30 miles to "E" and the only gas station I could find was Circle "K". So I filled up and went on my way. Car sits for about 5-6 hours and then I drive home, and after about 10 min my check engine light comes on. I run the codes and the lean code is back...after a month of being off. I wanted to know if anyone else has experienced this, before I go and start testing and replacing 02 sensors.