General consensus is that you should change 4-lead O2 sensors about every 80k miles. When I started to do this, I looked but could not find a writeup on how to go about it, so here goes.
Decide which type of sensors you are going to buy. I bought Bosch universal sensors. Bosch also makes a sensor specific to the celica. The difference is that the celi-specific sensor comes pre-wired to the connector that plugs into the celi’s wiring harness. If you get the unviersal, you have to cut the connector off of the stock sensor and splice the four wires into the new sensor. Here’s the kicker… the Advance Auto here charges $70 for the universal sensor and $170 for the specific one. I’ll splice 4 wires for $100, thanks. Oh, and to clarify, you will need TWO sensors… the ‘00 GT uses the same sensor upstream and downstream.
Locate your O2 sensors. The ‘00 GT has two of them; one in front of the cat (upstream) and one behind the cat (downstream). I’m operating on the assumption that all of the 7th gen celicas are set up the same way.
This picture shows the upstream sensor (looks like a spark plug with wires hanging out).
This picture shows the hole where the downstream sensor lives.
I would reccomend removing both sensors before doing any other part of the process, as you might find that they are nearly impossible to remove. The front one came out ok, but I had to use a gun penetrating solvent to break the rear one loose. You will need a 22mm box-end wrench for these. On the safety side, Never get under a jacked-up vehicle that is not on jack stands unless you want a closed casket funeral TOMORROW!. It’s also a good idea to set your e-brake and chock the back wheels before jacking.
Once you’ve removed both sensors, unplug their connectors from the wiring harness. The connectors are located under the carpet near where the passenger’s left foot would be:
Run the connectors down through the frame (you’ll have to push out the gaskets). Now take the removed sensors and new sensors to a clean, well-lighted area to do your wiring.
This part highly depends on the wiring instructions of the sensor you purchased. The two black wires on the stock celica sensors are the heater wires, and the polarity of these does not matter. On the bosch sensors, these are connected to the white wires. The other two wires are matched based on the matching table provided in the instructions.
A few things I forgot to do:
Don’t forget to thread your gaskets back on before wiring the new sensor. I forgot to do this and there is no way to fix it short of cutting it apart and wiring it back up again. As you can see in the above picture of the front sensor, I also forgot to thread the insulation/protecting sleeve back onto the sensor’s wires before wiring it to the connector.
Good luck, I hope this helps, and ENJOY your increased gas mileage!