P0135 indicates the upstream oxygen sensor which is located on top of the exhaust manifold. It has a springy thing around the wire. I believe you have already located it from what you said. A special socket with a slit in it to accomodate the wire is used to remove / replace them. The Mechanic (very experienced) that did mine had a hard time, and although the manual said to remove it from underneath, he eventually got at it better from the top.
Might want to clear the computer and see if it comes up again - may not.
2007 GC SXT - Magnesium - S&G - 3.8L - 173,000 kms
2002 GC Sport - Stone White - 3.3L - 358,500 kms
2003 Jeep TJ Sport - 4.0L - 279,000 kms