What are the sensors immediately before and after the catalytic converter that attach to the exhaust? After hearing a rattle, I looked at the exhaust and found that the screws holding the manifold behind the converter were missing (corroded away). I can pull the sensor out too. This might explain my "check engine" light going on and off. Looks like the gasket between this manifold and exhaust is shot too.
Thanks in advance

They are either both heated oxygen sensors or an air/fuel ratio sensor and a HO2 sensor. To find out for sure, bring your VIN to a Toyota parts department and get a part number.

http://www.autoshop101.com/forms/h37.pdf