Your vehicle's clean performance and fuel efficiency is due in part to its exhaust system, including parts like the pollution reducing catalytic converter that is responsible for lowering vehicle emissions in every modern car.
Vehicle exhaust systems used to produce a wide range of toxic hydrocarbons that contributed to pollution in cities across the country. Catalytic converters are largely the reason this is not the case anymore. They use a catalyst that reacts with those toxic chemicals to break them down into gasses that are natural in the earth's atmosphere, like water vapor and carbon dioxide. In many areas, a carb approved catalytic converter is required to make sure the vehicle's converter performs to the requirements of the local environmental regulations.
As you might imagine, even a carb legal catalytic converter can malfunction if it is overwhelmed by too many exhaust fumes or by the wrong kind. To keep your catalytic converter in good shape, you need to keep your engine and exhaust system in good shape. When the engine burns lubricants or gets contaminated with fluids that should not be present, the makeup of the exhaust changes. This, in turn, changes the performance of the converter. In some cases, it can cause buildup that renders the converter inoperable. Immediately addressing issues with exhaust flow and engine performance will help your converter last as long as possible.
The most telling signs of a failed carb compliant catalytic converter are the scent and color of the exhaust. Malfunctioning converters fail to break down heavy gasses that produce dark exhaust fumes with a sulfur smell, like rotten eggs. Other unconventional scents like a sweet burning smell can also occur if the exhaust is polluted with compounds that the engine should not be burning.
You also need to be on the lookout for less obvious signs that could point to more general exhaust issues as well, like sluggish engine performance and reduced fuel efficiency. They can tell you something is wrong before the exhaust changes color or smell, giving you time to order a converter before it is obvious yours is malfunctioning.