First off it's very annoying driving with the 'Check engine light" on for days. So hopefully this solution will help those with their corollas and prizms. Usually around 100k if your light goes on? It may be that the fuel filter near the gas tank needs to be replaced. A diagnostic will be more accurate. I don't know how to change that yet but search the internet.

The other ones are either you stepped on the gas pedal too hard overworking the tranny which causes the CEL to pop up or if it pops up all the sudden? What you can do is remove the Mass Air Flow Sensor that is attached to the cover of the Air filter by removing two screws and removing the wire connector and carefully just work it out of the hole. The next thing is get a can of electronic cleaner which you can get it at Radio shack. All you need to do is spray the cleaner which has a bulb with a wired attached to it several times. Let it dry for a minute and put it back in and start the car. The CEL will disappear. If it doesn't? You'll need to get it checked out by a quality mechanic

Update from Jan 31, 2011: Here's another problem that arise at 136,000 miles with the prizm. The check engine light came on again, well apparently cleaning the the MAF for the second time didn't help so it was a good guess that it wasn't the problem causing the CEL to pop up again. This time after taking it to Autozone for a free diagnostic, their report was a bit..how should I say it....overwhelming. The report states that there was a misfire in one of the cylinder and would need a tune up. Well I just tune the dame car when I got it at 124,000, that didn't make sense to me. They also stated that the fuel was burning too rich which I suspected.

Update from Feb 1, 2011: I checked the internet for more info on the CEL and found out that the Oxygen sensors are the normally the culprit for this problem especially if you have around 100k plus miles. I ordered the front and back Oxygen sensors from Amazon which they have some really good deals and I suggest you get Denso brand which Toyota uses. After installing it and running my car for a week (try to give it a good run in the highway) the light was gone. Give it a run for 1 week because it needs to clean the current and old dirt and grime inside it, so be patient. Other than that, it should take care of the CEL for the Prizm/Corolla cars out there.

Good Luck. I hope this helps!