Skilbourn

My service engine soon light came on last week, so I took the car into my mechanic buddy who checked the code on it for free. It came back as a code for the left cat. I remembered having a letter from GM stating that there are known bad cats on my model 2000 Camaro SS, and if they go, take the car to a chevy dealer and they will fix it at no cost. Well, I called my local chevy dealer, and they said as long as they can diagnose the problem as being the left cat through the diagnostic code, then it will be free. If they hook it up to the machine and it doesn't throw a code for the cats, then I will have to pay the $94 for a diagnostic fee. All seemed good, but then my service engine light went off on Friday. It hasn't been on since. If the light is not on when I take it to Chevy, can they tell my cat is bad? I heard if the light isn't on, then they won't find anything wrong with the cat... Does the computer store the codes??? HELP ME OUT HERE!! I need to know if I should cancel that appt...