If your Catera is having idle problems, if it stalls out on you occasionally, etc., you could take it to the shop and ask them to fix it, and they would most likely want to replace something called the IAC, or Idle Air Control valve.
If you have tons of money sitting around, go for it. On the other hand if you are like most people and dont want to pay 400 bucks unless you have to, then read on.

The IAC controls the air going into the engine durring idle, thats pretty much all i know about what it does, and its pretty much unimportant unless you REALLY wanna know.

What you will need:
flathead screwdriver
some strong plyers
pocket knife
a wrench with star shaped bit (you might be able to use a flathead screwdriver if you have one that will fit)
business card
x-acto knife

When you open the hood, look on the right side of the intake manifold for a plastic case that is screwd onto the manifold and is connected to a computer cable. You should only find one, but if you somehow cant decide between two, this one is like a cylindercal shape. You might have to take the yellow oil cap off to have enough room to unscrew the valve, and when your done taking the IAC, try and get the gasket in one pice cause your gonna have to trace it on the business card and cut it out to make a new one.
Unplug the cable too.

Make the new gasket to start with....doesnt need to be perfect, but try and do your best.

Now, onto the IAC unit itself. Stick your pocket knife or flathead screwdriver where there is a little pin holding it together, and pry up and once you have the top half above the pin, twist it i think clockwise, not sure.get the two halfs seperated, clean everything off, put the odd-shaped washer back on, put it back together.....yeah, thats it....
reinstall it with the new gasket, plug it in, and start it up. Hopefully it worked, but dont think that these things are built to last forever...you may still have to buy a new one....but theres a good chance you wont.

The whole thing should take about 45 minutes of work total....in the freezing cold it took me a few hours because i had no clue what i was doing....but its really not that hard....good luck :-)