Long time prowler first post !!

My check engine light has been on and off now for the past week. I checked the code and it came back with code p0150 O2 sensor malfunction bank 2 sensor 1. I am not sure how to interrupt this error. Which sensor location is malfunctioning the front one closest to the engine or the back one? Thanks in advance for any help and please be easy I am still a newbie

Mike