I have an early 07 2500 4 wheel drive . One of the last of the 5.9s. It has 470000 miles on it. I have just recently noticed occasional grey smoke cruising down the highway with little or no load. The injectors were replaced at 300000 miles plus so I don't think they are bad. I noticed when I got this truck new the exhaust smelled like I was burning propane - always figured that was the catalytic converter. It doesn't smell like that now. How long does a catalytic converter last?