A rotten egg smell emanating from catalytic converters is common. It is usually at its worst with new cars, decreasing as the car gets more miles on it. It usually becomes unnoticeable at idle for cars more than a few months old. The presence of this smell does not necessarily indicate a problem. You may have a particularly sensitive nose. The smell is hydrogen sulfide gas that, along with oxygen, is a byproduct of the conversion generated by the catalytic converter. Excessive hydrogen sulfide can be an indication of too much unburned fuel getting through the engine, but if there is excessive unburned fuel, the Check Engine light would normally come on. A "bad" catalytic converter, assuming that means one that is not functioning, would actually result is less hydrogen sulfide.