The rotten eggs smell is the result of unburned gasoline vapors reacting with chemicals in the catalytic converter. Your air/fuel mixture is too rich. You may have a clogged air filter. If the air filter is not clogged there may be a problem with a sensor controlling the vehicles fuel injection system. If your vehicle has a carburetor then the choke plate may be stuck or you may have a vacuum leak. Dealers will always blame cheap gas rather than the catalytic converter. If you do a lot of short hop driving, you may need to blow out that cat with an Italian tuneup. First, make sure your air filter is in good condition, or replace it, it’s cheap. Then tank up with premium unleaded and get out on the superhighway or other open road where you can drive at sustained high speed to heat up the platinum catalyst and burn out the gunk.