The job of a catalytic converter is to take in the bad air (exhaust pollutants) from the engine and convert it into good clean air. The amount of bad air coming from an average engine before 2007 is around 3%, after that it has dropped to nearly 1%. Unless there is something wrong with the engine, the catalytic converter has little to do. If you remember your high school chemistry, a catalyst is not used up in the process of the chemical reaction. In theory a catalytic converter will last forever.
So why does it smell? That’s a good question. The short answer is that all gasoline contains a small trace of sulfur. When the exhaust stinks of rotten eggs, it is because the catalytic converter is being flooded with fuel and the amount of sulfur passing through has increased dramatically. Your catalytic converter is being overworked or cooked. What you now smell is large amounts of bad air being converted (to help reduce emissions) at a very high rate (far greater than the recommended 3%), hence the high concentration of sulfur. The catalytic converter is actually doing its job, but not for long. A melt down will soon occur because of the tremendous amount of heat being generated by the excessive chemical reaction – think of the heat generated by lighting a match.
What the rotten egg smell is really trying to tell you is that there is something wrong up ahead, from the engine. The engine is adding excessive amounts of fuel (bad air). A few possible causes are a leaking injector, malfunctioning fuel pressure regulator, slipped timing belt, coolant sensor out of range or lazy oxygen (O2) sensor. The key indicator is that (LTFT) Long Term Fuel Trims will be out of range. This over fueling, over time, will eventually destroy the catalytic converter. Keep in mind that catalytic converters don’t just die, they are usually murdered and the fault should be found before a new catalytic converter is installed.
The answer to a smelly catalytic converter is that it is working but is being over worked because the root cause (real problem) of over fueling (bad air) is flooding your catalytic converter.