[mrnickleye]

From diesel motors, sulphur in the fuel and phosphorus from anti-wear oil additives can gum up a converter and prevent it from working.

A gas engine cat will self-clean if the engine is kept in good running condition. What really kills a cat is too much blow-by, so oil coats up the inside; an overly rich condition for long periods, and mis-fires, which send raw gas into it and over heats it to the point of burning it up.