PurpleJen5

Can someone please help me??? I'm having a problem with my Mustang. For a while now, my "Check Engine" light has been coming on. My dad, who is a mechanic, hooked it up to a diagnostic machine and it came up that my "system is too lean" and something was wrong with my o2 sensor. We replaced the o2 sensor. My "Check Engine" light is still coming on. And, now it feels like it's having a hard time shifting at around 40 mph. My dad drove it, and he said it feels like the torque converter is not working correctly. He hooked it back up to the diagnostic machine and in the torque converter field, it kept switching to yes and no, meaning it was sensing my torque converter when it was just idleing and it shouldn't be. It also said my "system is too lean" and "o2s no switch." Has anyone ever had these problems??? Does anyone have any in-site as to what is going on??? Everyone who has looked at my car is stumped. Please let me know what you think and/or your experiences. Thanks!!!!