[RobXS]

It is not an o2 sensor. It is called an A/F ratio sensor. There is a heater that burns out in the sensor. It is a simple replacement. It is in front of the Cat on the passenger side. The nuts aren't the easiest to get to and just hope that they are not too badly corroded. If you can get them off, it is a 15 min job. If you have to fight with them, it could take a while. I would suggest getting new nuts and gasket along with the sensor. You can get the sensor from the dealer. On line dealers like sponsors of this site such as Toyota of Dallas will get you the best price. It should run around $150 for the sensor and another $15 for the gasket and the nuts. Avoid after market parts. You may still have problems.