if you do it from under....you will need a 27mm deep socket that has been shortened about a quarter inch. I had to take a side grinder and trim some off of a brand new socket....but it worked.
From under, just go up the drivers side of the motor and you will see a small heat shield with three bolts. 2x 10mm, and one 13mm. The ICP sensor is right underneith the Y-pipe.
If you have any questions, shoot me an email....like I said I just replaced mine 3 days ago, so this stuff is fresh in my head......good luck!
nicholaseduncan@hotmail.com