Today my Fiance and I replaced the upstream o2 sensor in my 98' ti. I know there aren't a lot of Auto drivers out there, but I wanted to make mention of the work we did because it was not as straight forward as it comes across in the manuals, tutorials and other sources on the internet.
The Upstream O2 sensor is to the right of the transmission pan, making it an extremely tight fit. We tried several times to work the O2 socket onto the old O2 sensor with no luck. The socket was modified, about 1/2 cm taken off each side of the slit opening which ended up working. The tool used to modify the socket was a Dremel cut-off wheel (we went through 2.)
Since we did the replacement at home, the car was lifted and a jackstand was put under the passenger side tire. It took several shots of liquid wrench and leg leverage to get the old one off.
The flippin' thing was really on there.
Once it was off, we found that the replacement OE fit Bosch O2 (bought from Pep Boys) was the wrong one put into the right box...but AutoZone luckily carried the right O2 sensor. Seems locally there aren't many stores that carry the right O2 sensor. From my searching AutoZone and Pep Boys had the lowest price----DO NOT GET Universal though, as it probably won't fit into the harness...which is what some sly punk swapped my part out for.
Hopefully this might be helpful to other people.
98' Ti Upstream O2 sensor replacement. 
Linear Mode
