84 cadi: failed emissions?

my 84 deville failed MO state emissions....need thoughts on what the problem is. HC (hydrocarbons) read 1.5544grams/mile should be under 0.8000 NOx(oxides of nitrogen) read 5.9934 should be 2.0000g/m....i tried a fuel additive and it has fresh plugs and fluids but still failed. i figure it's the catalytic converter but just figured i'd get some more opinions