UMblkbr

I read a few ODBII codes on my wife's 2005 xA that had to do with evap system problems and an O2 sensor voltage problem. I checked the resistance on the front sensor and it was a short. I replaced it and the codes went away, briefly. Then, I got two more codes (P0037, P0137 I think) related to the rear O2 sensor. Here's my question: is it likely that both of the O2 sensors went bad (after ~84k miles) or is there another problem in the emission system that could be triggering these codes?