How to fix po135, code in a 2001-Silo.?

When I did my first tune-up for this van (http://www.oldsmobileforums.com/forums/showthread.php?p=45435#) was not easy (the back wires/sparkplugs are very hard to deal). now dealing with this sensor is even worst.
Questions:
-What are the symptoms, for this code?
-What would the correct way to get to that sensor?

Thank you.
http://www.oldsmobileforums.com/forums/images/misc/progress.gif

The easiest way to get at the B1S1 O2 sensor is up from the bottom. There is just enough room between the exhaust pipe and the body to get a hand in there. A 7/8" wrench is all that is needed. Put the van up on ramps, or a lift if you have one. Let it cool down for a few hours, then put your hand up there and unscrew the sensor. Your problem, most likely, is the sensor itself. However, it could be the circuit but that is unlikely unless a wire was burned through or something like that.

a bit late, thank you for the information, your instructions was correct.