Oxygen sensors

They're are two. The front one is readily accessible from the front of the car. It is located in the exhaust manifold. Sort of looks like a spark plug with a thin wire attached to it.

The second is located either at the end of the catylitic convertor or the pipe right after it. A little tougher to acccess.

Both can be change but I would recommend purchasin the special socket for each of them. Some auto parts stores even rent or loan them for free.

How do you know they need to be replaced?

:)