Hey Tacoma guys and gals!
I've recently replaced the o2 sensors on my Tacoma so I've decided to post a step-by-step procedure on how to do it.
Creeper or blanket
Extractor socket set with small hammer (optional)
3/8-inch drive ratchet
3/8-inch drive 10 mm socket
Upholstery tool or plastic plug puller
Philips head screw driver
8 by 1.25 re-threader or die
2 new 10 mm nuts with 8 by 1.25 thread and pitch
2 new lock washers 8 mm pitch
1) Determine which oxygen sensor you're replacing. This may be the most challenging part of the job. You may find a garage that will scan and diagnose the engine code for you inexpensively or even free of charge. This will determine which bank and whether upstream or downstream. If you're replacing them all, just be sure to place the correct ones in the correct banks. Many Tacomas use the same oxygen sensor up and down stream.
2) Using the creeper or blanket, crawl underneath the truck and locate the oxygen sensor you're replacing. Locate the plug of the sensor and use the straight-edged screwdriver to depress the lock and unplug. You may need to use the upholstery tool or plastic plug puller or even the straight-edged screw driver to release the plug from a manufactured drilled hole and plug affixing the wire to the vehicle. If the sensor you're replacing is the upstream one, you may also need to pull out a rubber plug, which runs into the passenger seat floorboard area. If this is the case, you will need to open the passenger side door and extract the bottom door molding with a Philips screwdrive and lift the carpet to locate the plug.
3) Locate the area where the oxygen sensor is bolted to the exhaust pipe. You will need to determine the condition of the nuts holding it on. It is not uncommon for these nuts to deteriorate and need an extractor socket pounded onto them and loosened with the 3/8-inch ratchet. Once the extractor is on securely, they generally break free relatively easy. If you do not need the extractor socket, simply use a 10 mm socket to remove the nuts.
4) Remove the oxygen sensor, and be sure to remove the old gasket and replace with a new one. Most new quality oxygen sensors come with them right in the box. For instance, the o2 sensor I bought from toyotapartszone.com was OEM (and cheap!) so the gasket was included. It is recommended that you use a 8 by 1.25 re-threader or die to clean the threads on the bolts of the sensor flange to ensure they do not snap when placing on new nuts.
5) Place new gasket onto bolts. Place sensor on top of new gasket. Replace with new 10 mm 8 by 1.25 pitch, and thread nuts and lock washers; tighten.
6) Plug sensor back in to plug and refasten to frame or replace door molding and floor plug.
Hope this helps someone out there!