P0137 weird problem...

Greetings everyone. I need some help figuring out my prius problem.

I have a 2008 prius, just purchased. It was in a little left side front fender bender but seems to be driving ok.

I cant register it yet because the CEL is on with a PO137 code. Bank 1 sensor 2 low voltage code. I have a obd II reader that can display live data too. Anyway, the voltage from the downstream o2 sensor was reading a constant .015v. The upstream sensor voltage is around 3.5v. From what i've read, this is well below the 0.1-0.9v range the downstream o2 sensor should be registering. So, i replaced the downstream sensor .

When i took it for a drive, after a few minutes of driving, the voltage on the downstream o2 sensor was reading in the proper range!! Yay!! However, as I continued driving the car, the o2 voltage sharply dropped back down to 0.015v and stayed there! When i stopped the car and let it cool down for about 45 min or so, i drove it again. After a few min, the sensor had voltage between .1 and .9v!! Then after a few minutes, it dropped back down to .015v and stayed there....

Any idea whats going on?! Help pls!!