Toyota Repair: Air fuel Sensor, air fuel ratio, light indicator

Question
My Check engine or Malfunction light indicator was lit up on my Sienna 2001 with 40,000 Miles last Thu 05/27/04. I took it to a dealer and they replaced it. It costs me 165 for sensor, 210 for labor and another 200 for work on fuel injection.

It came on again yesterday and I tighten the gas cap last night. I does not lid up this morning.

Do I have to have it checked by the dealer again? Is it the sensor the problem to begin with?
Thanks  

Answer
A loose gas cap can turn on the check engine light, this is not related to the air fuel ratio sensor problem that happened before but I don't know what the code is now so I'm guessing that it may have been the cap.
The reason that the light remains off after tightening the gas cap is that the computer has two trip detection logic, this means that if it sees a problem one time but is corrected it stores a pending code,light goes off, if the same problem happens again under the same conditions the check light will remain on.