The catalytic converter is an essential component in the exhaust system of an automobile. Catalytic converters serve the purpose of converting exhaust fumes into a less harmful form of gas. Without a catalytic converter on a car, the exhaust fumes from an engine would be full of carbon monoxide. Instead, when exhaust emerges from the tailpipe of a car equipped with a catalytic converter, the exhaust is primarily composed of carbon dioxide.
Check your warning lights. If your car was manufactured after 1996, it will be equipped with an on-board diagnostics system, or OBD-II. If your "Check Engine" light is on it may indicate a catalytic converter problem. If a light is on, you either need to take your car to a professional or use your own OBD-II tool to diagnose the code to see if it is a catalytic converter problem.
Check the vacuum pressure on the intake manifold. Attach a vacuum gauge to a vacuum port on the intake manifold. The intake manifold sits on top of the engine, and the vacuum port will look like a small protruding plug. You will have to look for the port as the location can vary based on the model of your car. Check the pressure while the vehicle is idling. Have an assistant rev the vehicle to around 2,500rpm. Within a short period, you should see the pressure return to a figure close to the one you noted at idle. If the pressure does not come back up, or continues dropping, there is likely a problem with the converter.
Visually inspect the catalytic converter itself. To do this, you have to disconnect and remove the converter from the exhaust system. It will be the larger item on the exhaust path on the engine side of the muffler. This is easiest to do if you can put your vehicle up on a lift. Normally, you should be able to shine a light through a catalytic converter and see it coming out the other end. If you can't see a light shone on a converter from the other end, it means that it is plugged. While you can replace it, the greater issue is that something is malfunctioning elsewhere in your system and is causing the plug. Also, when visually inspecting a converter if there is any loose material or if it rattles, then it needs to be replaced.
A malfunctioning catalytic converter is not something you can afford to ignore. Your fuel mileage may drop considerably with a non-functional converter. Some cars may not even start if the catalytic converter is not working. Acceleration can also be compromised.