Gymrat

I have a 2001 CL-S. About four months ago my car threw the p0131 code for the O2 sensor bank 1 sensor 1. The idle was rough when I was at a stoplight but other that that there weren't any major driving issues. I bought a Denso from Amazon and I had it changed. Only after about four or five weeks, the code came back and the idle was rough again. I bought another Denso from Amazon and had it changed again. The code came back only after a month of changing the sensor. The idle is a bit rough but there aren't any major issues. Has anybody ran into this issue? Thanks for your help.