Whats up with my upstream o2 sensor (I think bank 1) its throwing the P0135 code on 2011 Subaru Impreza WRX

I haven't changed my o2 sensor yet, but I have got the code reset twice just to double check what code was showing but every once in a while my check engine light will go off by itself and I was just curious to see why it would be going off by itself, it usually comes back on either shortly after the CEL goes off or when I start my car again.

Asked by for the 2011 Subaru Impreza WRX
poss under your 8yr 80k emission warranty at the dealer