Just finished this job and here's a few tips to add to the previous posts on replacing this sensor.
1. You only need to remove the smaller back, plastic, under carriage panel. Leave the larger, front one intact.
2. While its called the "front sensor", it is actually located near the oil pan drain plug on the drivers side. So it is quite in the rear of the engine area.
3. I placed the car on ramps which lifted it about 10", better to raise it a minimum of 12" using jack stands. (I use 20 ton stands as I am paranoid.)
4. The working area is very tight, and the plug connector for the sensor is ridiculously positioned relative to decoupling it. I truly suggest getting quotes on having the job done, as it is that difficult to pull off lying on your back with minimum clearance (10")
5. The female portion of the connector (this is the one attached to the frame of the car) is a softer rubber like material. I managed to squeeze it enough to create a gap in order to use a very small flat head (but long in length) screwdriver and managed to release it. But first, I pried this female portion loose from the frame of the car so that I could face it towards me, and using a headlamp, was able to spot how to free the male portion (which is a part of the o2 sensor) from the plug. It is ridiculously designed relative to the space afforded for releasing it.
6. The sensor body uses a 22mm socket designed for o2 sensors. This is a must have! I have set a that included a deep set socket grooved for the cable and one other socket that is offset for a 3/8" wrench. The space is tight, so I ended up using the deep socket attached to a 3/8" breaker bar. The head on a 3/8" socket wrench is just too big for the space but the smaller head on a 3/8" breaker bar made is possible. It is still very tight and I could see no way of using a torque wrench in that small space.
7. Some jobs are just not worth the trouble. I found an OEM O2 sensor on EBay for $125. In hindsight, I would first get quotes from mechanics to see if it is worth the trouble. I rebuilt the entire front end of my 2004 Ls430 and that was "doable". This O2 sensor is a joke, and simply poor OEM design that afforded no thought to replacement. You have been forewarned!