Toyota Repair: Toyota Corrolla P0420, toyota corrolla, emission control system

Question
In my car a light appeared saying check. I took my car to a friend that is a mecanic. He conected it to a machine. The machine gave out the Code P0420. He told me maybe the catalytic converter had a problem and that he would look up the code in the computer and tell me tommorrow what could be wrong. My question is can I still drive my car in this condition. How does it affect my car if I dont get this fix. He told me that I could drive to miami and nothing would happen to the car and that it just has to do with consuming more gasoline.Once he put in the machine the light went off. The check is no longer there. I have another question? Do you think the manufacture still has warranty on this issue. Would I be able to go to a dealership and tell them that that light went on and a friend checked it and the light went on and gave the code P0420 for them to apply warrant or should I wait untill the light comes up again. Will it come up again? O should I just get it fixed with my fried mechanic  

Answer
The federal emission control warranty period for catalytic converters is 8yrs/80,000 miles if you live in the US. If you go to the dealer they will start by diagnosing the system  and will charge a diagnostic fee to start, they don't care about any previous diagnosis or what someone else told you, they have to start new. The light will probably come on again later, is it safe to drive? YES. It is part of the emission control system and some reduction in fuel mileage and performance may be noticeable.