01 Pathfinder engine codes

[DRYGSXR]

I've got an 2001 Pathfinder SE with an illuminated SES. I pulled the codes and got the, all to popular, P1140. I know what that one is as it's been discussed in many previous threads and I ordered the VTC online to replace myself. The other code is P0158 and there is only one other thread which discusses this particular code. It has something to do with the O2 sensor but I don't know which one (how many O2 sensors are there on the Pathy?). Could I fix it myself if I bought a new sensor (I hate paying lots of money to Nissan for something that I can do myself. ). Thanks in advance for any information.