Having never looked at the working data of an a/f sensor on a 4unner, I will guess that it has gotten old and become lazy, meaning it isn’t reading the fuel trims correctly. Greg probably isn’t getting the best gas mileage at the moment.
This isn’t perfect, but gives you an idea of a lazy 02 sensor....
http://https://m.youtube.com/watch?v=jRa752CeNvg
If you have access to an older version of Windows on a laptop, you get Toyota’s Techstream off the Web and get a connector through Amazon. If you work on other people’s newer autos, this one works well for the money and the software can be upgraded......
http://https://www.auteltech.com/Automotive%20Diagnostic&%20Analysi/557.jhtml