Smell from Catalytic Converters is from 1-sulphur in the fuel and 2-overly rich A/F-i.e. poor tuning. Cats are purely reactive, they don't add anything. The Cats that trip the P0420 codes are just crap, you might as well just straight pipe 'em. The reason they trip the lights is that they scrimp on the expensive Platinum and Palladium.