1990 Riviera Code 44 won't go away

A month ago, the "Check Engine Soon" came on. I pulled Code 44 (C) & Code 44 (H), so this had been going on for awhile. According to my manual, Code 44 is "O2 sensor lean". So I replaced the sensor. At the next fill up, I also added some Techron. Seemed like the car ran much better, especially going up the mountains. But after another week, the "Check Engine" light came on again along with the Code 44 showing up as well.

I did disconnect the battery while installing the new sensor to clear the codes. Any ideas??

Thanks