How do you replace the O2 sensor on a 2003 Toyota Echo?

Answer 1

There are two of them, one before the cat, and one after. Normally the one that needs replaced is the one BEFORE the cat (closer to the engine). You should unplug it first, use a flashlight to locate the tab, pry the tab UP with a very small flathead screwdriver, then pull it out of the socket. After that you can unbolt the O2 sensor using a wrench or better yet a special tool built for that purpose (some autoparts stores will loan it to you for free). It can be difficult to unbolt, you may need to use a breaker bar and hammer. The new one is installed in the reverse order. If you buy a generic one to save money, you will most likely need to cut the wiring harness off of the old one and solder the wires to the new one - refer to the instructions that came with your sensor.