The car has 120k miles. Since a month ago the engine light has been on, with code P0420. I brought it to autoshop, and was told that the code means catalyst converter is broken and it costs $800 to replace it. They said there is nothing abnormal except this and it is not worth replacing it, so they just reset it. They also said it is because of using 87 gas for too long. They suggested switching to premium gas. The trouble is that there may be issue at state inspection if the light turns on again later, and their suggestion is to reset the code one or two months before the inspection to build up history. Anyone has similar issues?