02 Maxima O2 Sensor error code P0139
 

Hello I am having some trouble with my 02 Maxima, I keep getting error code P0139 from what people tell me I have to replace the O2 sensor Bank 1 Sensor 2, where is this located, is it in front on the catalytic converter or after it, or in the front or back of the exhaust system? I dont want the mechanic to touch it yet because he always end up doing more work on the car then what I took it to him for and then throws a huge bill at me. I have no idea what I am doing, I have had the car for 3 years, great car no complaints, just the normal wear and tear is all I have had to do to the car. The "service engine soon" light keeps lighting up, it will be on for a week or so then it will turn off on it's own for another 2 weeks before it comes back on. I don't know what to do. Any help would greatly be appreciated.