With the Downstream O2 giving high voltage is that leaning out the fuel or increasing?

Does the downstream O2 adjust the fuel/air or does it just monitor how well the cat is working and report for the emission?

 

The downstream sensor is just there to measure cat efficiency. It has no effect on A/F ratio.

 

Since I'm getting the high voltage on the downstream O2, does that mean that the upstream O2 is having issues?

 

I had the over voltage code for a downstream o2 sensor and no other codes. I replaced the downstream and started up to check if it did anything for the performance (which it didn't) and right away I got a new code for the upstream sensor. Replaced that and all is good now. I would go ahead and change it and see what happens.