I've been having a lot of white smoke coming off my Xterra. Usually only when it's cold or there's a lot of moisture. I can crank it up and the smoke flys out for a good 3 minutes with a huge cloud of smoke. I've also noticed the front on the passenger side has some smoke coming out too. Right around where that O2 sensor plugs in. I went by Oreily's and got a P0158 code. I would be happy to replace the O2 sensor to try and fix all of this. Only problem is I don't know which one it is! Engine-codes.com says P0158 refers to the Rear Heated Oxygen Sensor 2. Oreily's sells them as upstream and downstream so I don't know the difference. So could someone help me figure out what O2 sensor to replace and which one to buy???